Process for producing expanded polyolefin resin particles and expanded polyolefin resin particles

ABSTRACT

A process for producing expanded polyolefin resin particles with use as a foaming agent of water contained in an aqueous dispersion medium. The process includes dispersing polyolefin resin particles together with the aqueous dispersion medium into a closed vessel; heating the polyolefin resin particles up to or above a softening temperature of the polyolefin resin particles and pressurizing the polyolefin resin particles; and releasing the polyolefin resin particles into a zone whose pressure is lower than an internal pressure of the closed vessel. The polyolefin resin particles are composed of a polyolefin resin composition including polyolefin resin, polyvalent alcohol and a foam nucleating agent.

TECHNICAL FIELD

The present invention relates to processes for producing expandedpolyolefin resin particles and to expanded polyolefin resin particlesproduced by such processes. More specifically, the present inventionrelates to a process for producing expanded polyolefin resin particlesthat can be suitably used, for example, as raw materials for in-moldexpanded molded products and to expanded polyolefin resin particles.

BACKGROUND ART

Conventionally, there has been known a process for producing expandedparticles by dispersing polyolefin resin particles together with afoaming agent into an aqueous dispersion medium, impregnating the resinparticles with the foaming agent at a constant pressure and constanttemperature after raising the temperature, and then releasing them intoa low-pressure atmosphere. As for the foaming agent, disclosed examplesof such processes include processes that involve the use of volatileorganic foaming agents such as propane and butane (e.g., PatentLiterature 1) and processes that involve the use of inorganic gassessuch as carbon dioxide, nitrogen, and air (e.g., Patent Literatures 2and 3).

However, the volatile organic foaming agents are substances that aregreater in global warming potential than carbon dioxide, and as such,they are not environmentally preferable. Further, the volatile organicfoaming agents, such as propane and butane, have a capacity toplasticize polyolefin resin and therefore make it easy to attain a highexpansion ratio, but because their plasticizing capacity is great, theytend to make it difficult to control the expansion ratio and crystalcondition of expanded particles. Further, the volatile organic foamingagents are flammable substances, and as such, they make it necessary tomake facilities explosion-proof, thus incurring high costs infacilities.

Meanwhile, use of the inorganic gases, such as nitrogen and air, resultsin the incapability of, even under high pressure, attaining an amount ofimpregnation sufficient for a higher level of expansion, due to theirvery low capacity to spread completely through polyolefin resin.

In order to overcome these disadvantages, processes that involve the useas a foaming agent of water used as a dispersion medium have beenproposed as processes for economically producing expanded polyolefinresin particles that can be suitably used for production of in-moldexpanded molded products.

Proposed as a process that involves the use of water as a foaming agentis a process for producing expanded crystalline polyolefin polymerparticles by: dispersing crystalline polyolefin polymer particlescontaining 10 to 70% by weight of an inorganic filler into water, whichserves as a dispersion solution, in a closed vessel; impregnating thecrystalline polyolefin polymer particles with the water, which serves asa dispersion medium, at a pressure of not lower than the saturated vaporpressure of the dispersion liquid and a temperature of not higher thanthe melting point of the crystalline polyolefin polymer particles, whilekeeping the dispersion liquid in a high-pressure zone under suchtemperature conditions that the crystallization of the polymer particlesprogresses; and then releasing the dispersion liquid into a low-pressurezone (e.g., Patent Literature 4). However, the expanded particles thatare obtained by this process contain the inorganic filler in largeamounts, and therefore are extremely small in cell diameter and tend tohave a higher open cell ratio. As such, they are not sufficient infusion, surface appearance, and mechanical properties such ascompressive strength when processed into in-mold expanded moldedproducts.

Further proposed is a process for producing expanded polyolefin resinparticles by: dispersing polyolefin resin particles containing either anaqueous inorganic substance or a hydrophilic polymer into water in aclosed vessel; heating the resin particles up to or above the softeningtemperature of the resin particles to turn the resin particles intohydrous polyolefin resin particles; and then releasing the dispersionliquid into a low-pressure zone (e.g., Patent Literatures 5 to 10).According to the descriptions, this process makes it possible to obtainexpanded polypropylene resin particles with a high expansion ratio underlow pressure in the vessel while using water, carbon dioxide, nitrogen,or the like, which are environmentally friendly, as a foaming agent.

However, in the case of such polyolefin resin particles containing awater-soluble inorganic substance as described in Patent Literature 6,an increase in amount of addition of the water-soluble inorganicsubstance for the purpose of increasing the expansion ratio or, inparticular, making the expansion ratio eight times or more as high tendsto result accordingly in extremely smaller cells. In consequence,in-mold expanded molded products made from the resultant expandedparticles suffer from such problems as lower fusibility between expandedparticles, thus bringing about degradation in commercial value of themolded products and deterioration in the rate at which the moldedproducts are produced. Further, since the expansion ratio and the celldiameter change in conjunction with each other, there are problems ofdifficulties, for example, with adjusting (controlling) only theexpansion ratio with the cell diameter held constant, which imposeslimitations on production of expanded particles having desiredproperties.

Expanded polypropylene resin particles produced with use of carbondioxide with the moisture content of such polypropylene resin particlescontaining a hydrophilic polymer as described in Patent Literature 7held at 8% by weight or higher suffer from the disadvantage of tendingto shrink immediately after foaming due to their high moisture content.

Furthermore, although the processes of Patent Literature 5 to 7 forproducing expanded polyolefin resin particles containing a hydrophilicpolymer make it possible to obtain expanded polyolefin resin particleswith a high expansion ratio under low pressure in the vessel while usingenvironment-friendly water as a foaming agent, cells in the resultingexpanded particles tend to be extremely small or nonuniform. In-moldexpanded molded products made from the resultant expanded particles witha low expansion ratio have no particular problems. However, those madewith a high expansion ratio under such molding conditions as a shortmolding cycle and a short curing time after molding in recent pursuit oflower production costs have such problems as wrinkles on their surfacesand distortion in their shapes due to their large dimensional shrinkage,thus bringing about degradation in their commercial value anddeterioration in the rate at which they are produced.

Such polyolefin resin particles containing a hydrophilic polymer havethe hydrophilic polymer added so that water is used as a foaming agent.However, because in general a hydrophilic polymer is poor indispersibility in polyolefin resin particles, there are suchdisadvantages as occurrence of variations in expansion ratio of theexpanded particles and proneness to poor fusion between expandedparticles processed into in-mold expanded molded products.

Meanwhile, there is disclosed a process for producing expanded particleswithout extremely smaller cells by introducing carbon dioxide as afoaming agent into polymer particles containing a polypropyleneglycol-polyethylene glycol polymer together with an inorganic substance(e.g., Patent Literature 11). Due to the low compatibility of thepolypropylene glycol-polyethylene glycol polymer with polypropyleneresin, the process tends to cause troubles such as occurrence of strandbreakage due to poor dispersion in the step of preparing the polymerparticles and fluctuation in the feed of a molten resin in an extruder.Therefore, the polypropylene glycol-polyethylene glycol polymer can onlybe added in a minute amount, and due to its low water absorption rate,there has been no choice but to resort to foaming by carbon dioxide.Further, because the polypropylene glycol-polyethylene glycol polymerused has a great average molecular weight, it has been necessary to usepolypropylene resin having a melt index of not less than 10 g/10 min, soas to obtain expanded polypropylene resin particles having a highexpansion ratio. Furthermore, there have been such disadvantages asproneness to a lower rate of fusion between expanded particles processedinto molded products, degradation in heat resistance, and degradation instrength.

CITATION LIST

Patent Literature 1

-   Japanese Patent Application Publication, Tokukosho, No. 56-1344 B

Patent Literature 2

-   Japanese Patent Application Publication, Tokukohei, No. 4-64332 B

Patent Literature 3

-   Japanese Patent Application Publication, Tokukohei, No. 4-64334 B

Patent Literature 4

-   Japanese Patent Application Publication, Tokukosho, No. 49-2183 B

Patent Literature 5

-   Japanese Patent Application Publication, Tokukaihei, No. 3-223347 A

Patent Literature 6

-   International Publication No. WO 98/25996

Patent Literature 7

-   Japanese Patent Application Publication, Tokukaihei, No. 10-152574 A

Patent Literature 8

-   Japanese Patent Application Publication, Tokukaihei, No. 10-298338 A

Patent Literature 9

-   Japanese Patent Application Publication, Tokukaihei, No. 10-306179 A

Patent Literature 10

-   Japanese Patent Application Publication, Tokukaihei, No. 11-106576 A

Patent Literature 11

-   Japanese Patent Application Publication, Tokukaihei, No. 5-163381 A

SUMMARY OF INVENTION Technical Problem

It is an object of the present invention to provide: a process forproducing expanded polyolefin resin particles whose cells are not madenonuniform or extremely smaller as seen in the conventional expandedparticles, which vary less in both cell diameter within each expandedparticle and expansion ratio from one expanded particle to another,whose cell diameter and expansion ratio can be easily controlledindependently, and which, when subjected to in-mold expansion molding,give in-mold expanded molded products satisfactory in fusibility andsurface properties and high in dimensional accuracy. It is anotherobject of the present invention to provide expanded polyolefin resinparticles that are obtained by such a process.

Solution to Problem

As a result of their diligent study to solve the foregoing problems, theinventors found that expanded polyolefin resin particles whose cells arenot made nonuniform or extremely smaller as has been the caseconventionally, which vary less in cell diameter within each expandedparticle, and whose cell diameter and expansion ratio have been adjustedcan be produced with a low environmental load by: impregnating, withwater, polyolefin resin particles composed of a polyolefin resincomposition containing a particular substance and a foam nucleatingagent; and expanding the polyolefin resin particles.

Furthermore, the inventors also found that concomitant use of carbondioxide as a foaming agent in addition to the water makes the cells moresatisfactory in uniformity and makes it easy to increase the expansionratio.

That is, the present invention is as follows:

(1) A process for producing expanded polyolefin resin particles with useas a foaming agent of water contained in an aqueous dispersion medium,the process including the steps of: dispersing polyolefin resinparticles together with the aqueous dispersion medium into a closedvessel; heating the polyolefin resin particles up to or above asoftening temperature of the polyolefin resin particles and pressurizingthe polyolefin resin particles; and releasing the polyolefin resinparticles into a zone whose pressure is lower than an internal pressureof the closed vessel, the polyolefin resin particles being composed of apolyolefin resin composition including: polyolefin resin; polyethyleneglycol in not less than 0.05 parts by weight to not more than 2 parts byweight to 100 parts by weight of the polyolefin resin; and a foamnucleating agent.

(2) A process for producing expanded polyolefin resin particles with useas a foaming agent of water contained in an aqueous dispersion medium,the process including the steps of: dispersing polyolefin resinparticles together with the aqueous dispersion medium into a closedvessel; heating the polyolefin resin particles up to or above asoftening temperature of the polyolefin resin particles and pressurizingthe polyolefin resin particles; and releasing the polyolefin resinparticles into a zone whose pressure is lower than an internal pressureof the closed vessel, the polyolefin resin particles being composed of apolyolefin resin composition including: polyolefin resin; polyvalentalcohol in not less than 0.05 parts by weight to not more than 2 partsby weight to 100 parts by weight of the polyolefin resin, the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups; and a foam nucleating agent.

(3) A process for producing expanded polyolefin resin particles with useas a foaming agent of water contained in an aqueous dispersion medium,the process including the steps of: dispersing polyolefin resinparticles together with the aqueous dispersion medium into a closedvessel; heating the polyolefin resin particles up to or above asoftening temperature of the polyolefin resin particles and pressurizingthe polyolefin resin particles; and releasing the polyolefin resinparticles into a zone whose pressure is lower than an internal pressureof the closed vessel, the polyolefin resin particles being composed of apolyolefin resin composition including: polyolefin resin; awater-absorbing substance in not less than 0.01 part by weight to notmore than 5 parts by weight to 100 parts by weight of the polyolefinresin, the water-absorbing substance having no function of formingfoaming nuclei; and a foam nucleating agent.

(4) The process as set forth in any one of (1) to (3), wherein thepolyolefin resin is polypropylene resin.

(5) The process as set forth in (4), wherein the polypropylene resin hasa melt index of not less than 2 g/10 minutes to not more than 9 g/10minutes.

(6) The process as set forth in any one of (1), (4), and (5), whereinthe polyethylene glycol has an average molecular weight of not less than200 to not more than 9,000.

(7) The process as set forth in any one of (1), (4), (5), and (6),wherein the polyethylene glycol has an average molecular weight of notless than 200 to not more than 600.

(8) The process as set forth in any one of (2), (4), and (5), whereinthe polyvalent alcohol having a carbon number of not less than 3 to notmore than 6 and three or more hydroxyl groups is one or more typesselected from among glycerin, diglycerin, pentaerythritol,trimethylolpropane, sorbitol, and D-mannitol.

(9) The process as set forth in any one of (2), (4), (5), and (8),wherein the polyvalent alcohol having a carbon number of not less than 3to not more than 6 and three or more hydroxyl groups is glycerin.

(10) The process as set forth in (9), wherein the glycerin is added innot less than 0.05 parts by weight to not more than 0.5 parts by weightto 100 parts by weight of the polyolefin resin.

(11) The process as set forth in any one of (3) to (5), wherein thewater-absorbing substance having no function of forming foaming nucleiis a compound having a polyalkylene oxide structure.

(12) The process as set forth in (11), wherein the compound having apolyalkylene oxide structure is a copolymer containing a polyolefinblock and a polyethylene oxide block.

(13) The process as set forth in any one of (3) to (5), wherein thewater-absorbing substance having no function of forming foaming nucleiis at least one type selected from among bentonite, synthetic hectolite,and synthetic zeolite.

(14) The process as set forth in any one of (3) to (5), (11), and (12),wherein the water-absorbing substance having no function of formingfoaming nuclei has a melting point of lower than 150° C.

(15) The process as set forth in any one of (1) to (14), wherein thepolyolefin resin particles are composed of a polyolefin resincomposition containing the foam nucleating agent in not less than 0.005parts by weight to not more than 2 parts by weight to 100 parts byweight of the polyolefin resin.

(16) The process as set forth in any one of (1) to (15), the processinvolving concomitant use of carbon dioxide as a foaming agent.

(17) The process as set forth in (3), wherein expanded polypropyleneresin particles having a volatile content of not less than 0.1% byweight to 7% by weight, an expansion ratio of not less than 8 times tonot more than 25 times, an average cell diameter of not less than 130 μmto not more than 500 μm, and a cell diameter variation of less than 0.4are obtained by: dispersing the polypropylene resin particles togetherwith the aqueous dispersion medium into the closed vessel; heating thepolypropylene resin particles up to or above the softening temperatureof the polypropylene resin particles; and releasing the polypropyleneresin particles into a zone whose pressure is lower than the internalpressure of the closed vessel, the polypropylene resin particlescontaining: polypropylene resin having a melt index of not less than 2g/10 minutes to not more than 9 g/10 minutes; a water-absorbingsubstance in not less than 0.01 part by weight to not more than 5 partsby weight to 100 parts by weight of the polypropylene resin, thewater-absorbing substance having no function of forming foaming nuclei;and a foam nucleating agent in not less than 0.005 parts by weight tonot more than 1 part by weight to 100 parts by weight of thepolypropylene resin.

(18) Expanded polyolefin resin particles that are obtained by a processas set forth in any one of (1), (4) to (7), (15), and (16), the expandedpolyolefin resin particles containing not less than 0.05% by weight tonot more than 2% by weight of the polyethylene glycol, the expandedpolyolefin resin particles having an expansion ratio of not less than 10times to not more than 45 times and an average cell diameter of not lessthan 50 μm to not more than 800 μm, the expanded polyolefin resinparticles having a crystal structure that exhibits two or more meltingpoints on a DSC curve that is obtained by raising a temperature of theexpanded polyolefin resin particles from 40° C. to 220° C. at a heatingrate of 10° C./min in differential scanning calorimetry.

(19) Expanded polyolefin resin particles that are obtained by a processas set forth in any one of (2), (4), (5), (8), (9), (10), (15), and(16), the expanded polyolefin resin particles containing not less than0.05% by weight to not more than 2% by weight of the polyvalent alcoholhaving a carbon number of not less than 3 to not more than 6 and threeor more hydroxyl groups, the expanded polyolefin resin particles havingan expansion ratio of not less than 10 times to not more than 45 timesand an average cell diameter of not less than 50 μm to not more than 800μm, the expanded polyolefin resin particles having a crystal structurethat exhibits two or more melting points on a DSC curve that is obtainedby raising a temperature of the expanded polyolefin resin particles from40° C. to 220° C. at a heating rate of 10° C./min in differentialscanning calorimetry.

(20) Expanded polyolefin resin particles that are obtained by a processas set forth in any one of (3), (4), (5), (11) to (16), the expandedpolyolefin resin particles containing not less than 0.01% by weight tonot more than 5% by weight of the water-absorbing substance having nofunction of forming foaming nuclei, the expanded polyolefin resinparticles having an expansion ratio of not less than 8 times to not morethan 45 times and an average cell diameter of not less than 50 μm to notmore than 800 μm, the expanded polyolefin resin particles having acrystal structure that exhibits two or more melting points on a DSCcurve that is obtained by raising a temperature of the expandedpolyolefin resin particles from 40° C. to 220° C. at a heating rate of10° C./min in differential scanning calorimetry.

(21) The expanded polyolefin resin particles as set forth in any one of(18) to (20), the expanded polyolefin resin particles having anexpansion ratio of not less than 10 times to not more than 25 times andan average cell diameter of not less than 130 μm to not more than 500μm.

(22) The expanded polyolefin resin particles as set forth in any one of(18) to (21), the expanded polyolefin resin particles having a volatilecontent of not less than 0.1% by weight to not more than 7% by weightand a cell diameter variation of less than 0.4.

(23) Expanded polypropylene resin particles including: polypropyleneresin; a water-absorbing substance in not less than 0.01 part by weightto not more than 5 parts by weight to 100 parts by weight of thepolypropylene resin, the water-absorbing substance having no function offorming foaming nuclei; and a foam nucleating agent in not less than0.005 parts by weight to not more than 1 part by weight to 100 parts byweight of the polypropylene resin, the expanded polypropylene resinparticles having a melt index of not less than 2 g/10 minutes to notmore than 12 g/10 minutes, a volatile content of not less than 0.1% byweight to not more than 7% by weight, an expansion ratio of not lessthan 8 times to not more than 25 times, an average cell diameter of notless than 130 μm to not more than 500 μm, and a cell diameter variationof less than 0.4.

(24) The expanded polypropylene resin particles as set forth in (23),the expanded polypropylene resin particles having a crystal structurethat exhibits two or more melting points on a DSC curve that is obtainedby raising a temperature of the expanded polypropylene resin particlesfrom 40° C. to 220° C. at a heating rate of 10° C./min in differentialscanning calorimetry.

(25) Expanded polyolefin resin particles (referred to as “expandedpolyolefin resin particles (P)”) that are obtained by a process as setforth in any one of (3) to (5) and (11) to (17), in comparison withexpanded polyolefin resin particles (referred to as “expanded polyolefinresin particles (Q)”) produced in just the same way except that thewater-absorbing substance having no function of forming foaming nucleiis not contained, the expanded polyolefin resin particles (P) having avolatile content and an average cell diameter that satisfy the followingformulas (E1) and (E2): (E1): Volatile Content of Expanded PolyolefinResin Particles (P)≧Volatile Content of Expanded Polyolefin ResinParticles (Q)×1.1; and (E2) Average Cell Diameter of Expanded PolyolefinResin Particles (P)≧Average Cell Diameter of Expanded Polyolefin ResinParticles (Q)×0.7, the expanded polyolefin resin particles (P) having amelt index of not less than 1 g/10 minutes to not more than 12 g/10minutes.

(26) The expanded polyolefin resin particles as set forth in any one of(18) to (22) and (25), the expanded polyolefin resin particles beingexpanded polypropylene resin particles.

(27) A polyolefin resin in-mold expanded molded product that is obtainedby filling a mold with expanded polyolefin resin particles as set forthin any one of (18) to (26) and heating the expanded polyolefin resinparticles.

Advantageous Effects of Invention

According to the present invention, a process for producing expandedpolyolefin resin particles with use of water as a foaming agent or, inparticular, with use of water and carbon dioxide as foaming agents makesit possible to obtain expanded polyolefin resin particles with a highexpansion ratio whose cells are made uniform without being madeextremely smaller and whose cell diameter and expansion ratio can beeasily controlled independently. Further, use of expanded polyolefinresin particles of the present invention makes it possible to obtainin-mold expanded molded products high in rate of fusion, small in gapbetween particles, shrinkage, and distortion, beautiful in surfaceproperties, and high in dimensional accuracy. In particular, in-moldexpanded molded products molded after increasing the expansion ratio ofexpanded polyolefin resin particles of the present invention bytwo-stage foaming are better than conventional ones molded with use ofwater as a foaming agent.

BRIEF DESCRIPTION OF DRAWINGS

FIG. 1 is an example of a DSC curve that is obtained by raising thetemperature of not less than 1 mg to not more than 10 mg of expandedpolypropylene resin particles of the present invention from 40° C. to220° C. at a heating rate of 10° C./min in differential scanningcalorimetry, where A is the point at which the smallest quantity of heatabsorption is reached between the two melting peaks of the DSC curve, Qhis the quantity of heat at the melting peak on the higher-temperatureside, which is the right one of the two areas surrounded by the DSCcurve and tangents to the DSC curve drawn from the point A, and Ql isthe quantity of heat at the melting peak on the lower-temperature side,which is the left one of the two areas.

FIG. 2 is a graph showing a relationship between the expansion ratio andthe average cell diameter in expanded polypropylene resin particlesobtained with varying amounts of additives to have different expansionratios.

DESCRIPTION OF EMBODIMENTS

A first aspect of the present invention relates to a process forproducing expanded polyolefin resin particles with use as a foamingagent of water contained in an aqueous dispersion medium, the processincluding the steps of: dispersing polyolefin resin particles togetherwith the aqueous dispersion medium into a closed vessel; heating thepolyolefin resin particles up to or above a softening temperature of thepolyolefin resin particles and pressurizing the polyolefin resinparticles; and releasing the polyolefin resin particles into a zonewhose pressure is lower than an internal pressure of the closed vessel,the polyolefin resin particles being composed of a polyolefin resincomposition including: polyolefin resin; polyethylene glycol in not lessthan 0.05 parts by weight to not more than 2 parts by weight to 100parts by weight of the polyolefin resin; and a foam nucleating agent.

A second aspect of the present invention relates to a process forproducing expanded polyolefin resin particles with use as a foamingagent of water contained in an aqueous dispersion medium, the processincluding the steps of: dispersing polyolefin resin particles togetherwith the aqueous dispersion medium into a closed vessel; heating thepolyolefin resin particles up to or above a softening temperature of thepolyolefin resin particles and pressurizing the polyolefin resinparticles; and releasing the polyolefin resin particles into a zonewhose pressure is lower than an internal pressure of the closed vessel,the polyolefin resin particles being composed of a polyolefin resincomposition including: polyolefin resin; polyvalent alcohol in not lessthan 0.05 parts by weight to not more than 2 parts by weight to 100parts by weight of the polyolefin resin, the polyvalent alcohol having acarbon number of not less than 3 to not more than 6 and three or morehydroxyl groups; and a foam nucleating agent.

A third aspect of the present invention relates to a process forproducing expanded polyolefin resin particles with use as a foamingagent of water contained in an aqueous dispersion medium, the processincluding the steps of: dispersing polyolefin resin particles togetherwith the aqueous dispersion medium into a closed vessel; heating thepolyolefin resin particles up to or above a softening temperature of thepolyolefin resin particles and pressurizing the polyolefin resinparticles; and releasing the polyolefin resin particles into a zonewhose pressure is lower than an internal pressure of the closed vessel,the polyolefin resin particles being composed of a polyolefin resincomposition including: polyolefin resin; a water-absorbing substance innot less than 0.01 part by weight to not more than 5 parts by weight to100 parts by weight of the polyolefin resin, the water-absorbingsubstance having no function of forming foaming nuclei; and a foamnucleating agent.

The polyethylene glycol in the present invention is a nonionicwater-soluble polymer having an ethylene glycol polymerized structure,and has an average molecular weight of approximately not more than50,000. It is preferable that the polyethylene glycol that is used inthe present invention have an average molecular weight of not less than200 to not more than 9,000, or more preferably not less than 200 to notmore than 600. In general, glycols are slightly inferior incompatibility with polyolefin resin. However, such polyethylene glycolhaving an average molecular weight of not less than 200 to not more than9,000 is dispersed comparatively satisfactorily even in the step ofkneading a mixture of the polyolefin resin and the polyethylene glycolin an extruder and producing the polyolefin resin particles by a strandcut method, and therefore tends to cause less troubles such asoccurrence of strand breakage and instability in the feed of the moltenresin. Furthermore, the resulting expanded polyolefin resin particlestend to have uniform cells and to vary less in expansion ratio. Anin-mold expanded molded product obtained by in-mold molding of suchexpanded polyolefin resin particles tends to be high in rate of fusion,beautiful in surface, and low in rate of dimensional shrinkage.

Selection of polyethylene glycol having such a low average molecularweight of not less than 200 to not more than 600 leads to an increase inimpregnation ability of carbon dioxide, which is suitably usedconcomitantly with water, and therefore tends to give expandedpolyolefin resin particles having a high expansion ratio.

Polyethylene glycol having an average molecular weight of more than50,000 is low in dispersibility into polyolefin resin, thus causinggreat variations in cell diameter within each expanded polyolefin resinparticle and decreasing the amount of water with which polyolefin resinparticles are impregnated. The resulting expanded polyolefin resinparticles tend to have a lower expansion ratio.

It should be noted that it is possible to concomitantly use polyethyleneglycol having a different molecular weight. However, such a similarsubstance to glycols as a polypropylene glycol-polyethylene glycolpolymer is poor in dispersibility into polyolefin resin, therefore canonly be added in a minute amount, and furthermore, due to its low waterabsorption rate, is not suitable for concomitant use. Further, althoughcross-linked polyalkylene oxide is commercially available, it incursvery high costs because it needs to be added in a large amount formoisture content and is an expensive substance.

It should be noted that the average molecular weight of polyethyleneglycol can be measured by using a liquid chromatograph mass spectrometersuch as an LCQ Advantage manufactured by Thermo Fisher Scientific K.K.

In the present invention, the polyvalent alcohol having a carbon numberof not less than 3 to not more than 6 and three or more hydroxyl groupsis such that the moisture content can be increased simply by adding thepolyvalent alcohol in a small amount into the polyolefin resin andexpanded particles having a high expansion ratio can be obtained.Specifically, examples of the polyvalent alcohol having a carbon numberof not less than 3 to not more than and three or more hydroxyl groupsinclude glycerin, 1,2,4-butanetriol, diglycerin, pentaerythritol,trimethylolpropane, sorbitol, D-mannitol, erythritol, hexanetriol,xylitol, D-xylose, inositol, fructose, galactose, glucose, and mannose,etc. Among them, it is preferable that the polyvalent alcohol having acarbon number of not less than 3 to not more than 6 and three or morehydroxyl groups be one or more types selected from among glycerin,diglycerin, pentaerythritol, trimethylolpropane, sorbitol, andD-mannitol, or more preferably glycerin. Glycerin is a substance that ishighly hygroscopic, is used as a food additive, and is drastically safeto the human body, which makes it possible to use expanded polyolefinresin particles of the present invention as raw materials for in-moldexpanded molded products that are used in food contact applications.Further, glycerin has low environmental impact because it is easilydecomposable even when eluted into discharged water during production.Glycerin is suitable also because it is easily available andinexpensive.

In general, a hydrophilic substance is slightly inferior incompatibility with polyolefin resin. However, the polyvalent alcoholhaving a carbon number of not less than 3 to not more than 6 and threeor more hydroxyl groups, as used in the present invention, is dispersedcomparatively satisfactorily even in the step of blending it with thepolyolefin resin, kneading the mixture in an extruder, and producing thepolyolefin resin particles by a strand cut method, and therefore tendsto cause less troubles such as occurrence of strand breakage andinstability in the feed of the molten resin. Furthermore, the resultingexpanded polyolefin resin particles have uniform cells and vary less inexpansion ratio. An in-mold expanded molded product obtained by in-moldexpansion molding of such expanded polyolefin resin particles is high inrate of fusion, beautiful in surface, and low in rate of dimensionalshrinkage.

In the present invention, the water-absorbing substance having nofunction of forming foaming nuclei refers to a substance that has nofunction of forming foaming nuclei and has water-absorbing properties.

The phrase “having no function of forming foaming nuclei” in the presentinvention refers to a substance that has a relationship “Average CellDiameter of (a)≧Average Cell Diameter of (b)×0.7” between expandedpolyolefin resin particles (a) containing the substance in 0.5 parts byweight to 100 parts by weight of the polyolefin resin and expandedpolyolefin resin particles (b) obtained by expanding the polyolefinresin particles under just the same conditions except that the substanceis not contained. The average cell diameter in this case is an averagecell diameter L(av) measured according to a method mentioned later.

In the present invention, the substance that has water-absorbingproperties encompasses, in general, substances that have water-absorbingproperties, hygroscopic properties, and solubility to or compatibilitywith water. Examples of such substances include a water-soluble polymer,a water-absorbing polymer, a hydrophilic polymer, a water-solubleorganic substance, a water-absorbing organic substance, a hydrophilicorganic substance, a water-soluble inorganic substance, awater-absorbing inorganic substance, and a hydrophilic inorganicsubstance, etc.

These substances are not particularly limited in water absorption rate.However, from the point of view of improving the expansion ratio of theresulting expanded polyolefin resin particles, it is preferable thewater absorption rate be not lower than 0.1%, or more preferably notlower than 0.5%. Such a water absorption rate is measured, for example,in conformity to ASTM D570.

In a conventional technique that involves the addition of awater-soluble inorganic substance or the like, it has been possible toincrease the expansion ratio of expanded particles by increasing theamount of addition of the water-soluble inorganic substance. However,because the water-soluble inorganic substance has a function of formingfoaming nuclei, a significant increase in number of cells is observedalong with an increase in expansion ratio, with the result that theaverage cell diameter is very small. In consequence, the cells tend tobe small in wall thickness. An in-mold expanded molded product obtainedby in-mold expansion molding of such expanded particles is low in rateof fusion and large in gap between particles, shrinkage, and distortion.

As opposed to this, since the present invention uses the water-absorbingsubstance having no function of forming foaming nuclei, there is nosignificant decrease in average cell diameter even when the expansionratio is increased by increasing the amount of addition of thewater-absorbing substance. An in-mold expanded molded product obtainedby in-mold expansion molding of such expanded particles produced by theprocess of the present invention is high in rate of fusion, small in gapbetween particles, shrinkage, and distortion to be good in appearanceand excellent.

The foregoing has described the water-absorbing substance having nofunction of forming foaming nuclei, as used in the present invention, interms of the absence of the function of forming foaming nuclei and interms of water-absorbing properties. Specific examples of thewater-absorbing substance having no function of forming foaming nucleiinclude such substances as below.

That is, the specific examples include: (A) compounds each having apolyalkylene oxide or polyethylene oxide structure such a copolymer(e.g., marketed as Pelestat by Sanyo Chemical Industries, Ltd.)containing polyalkylene oxide as a block portion, polypropylene glycol,polyethylene glycol, or cross-linked polyalkylene oxide; (B) hydrophilicpolymers such as sodium polyacrylate, cellulose, carboxymethylcellulosesodium, and polyvinyl alcohol; and (C) inorganic compounds such aszeolite, bentonite, and synthetic hectolite (Laponite).

The specific examples further include (D) surfactants such as: (i)cationic surfactants such as aliphatic amine salt, hydroxyalkylmonoethanolamine salt, and aliphatic quaternary ammonium salt; (ii)anionic surfactants such as alkyl sulfonate, alkyl benzene sulfonate,alkyl naphthalene sulfonate, sulfosuccinate, α-olefin sulfonate, N-acylsulfonate, alkyl sulfate, alkyl ether sulfate, alkyl aryl ether sulfate,alkyl amide sulfate, alkyl phosphate, alkyl ether phosphate, alkyl arylether phosphate, alkyl ether carboxylate, N-acyl amino-acid salt; (iii)nonionic surfactants such as alkyl and alkyl aryl polyoxyethylene ether,alkyl aryl formaldehyde condensed polyoxyethylene ether, polyoxyethylenepolyoxypropyl alkyl ether, polyoxyethylene ether of glycerin ester,polyoxyethylene ether of sorbitan ester, polyoxyethylene ether ofsorbitol ester, polyethylene glycol fatty ester, glycerin ester, higherfatty acid glycerin ester, polyglycerin ester, sorbitan ester, propyleneglycol ester, sucrose esters, aliphatic alkanol amide, polyoxyethylenefatty amide, polyoxyethylene alkyl amine, and amine oxide; and (iv)ampholytic surfactants such as carboxy betaine, imidazolium betaine, andaminocarboxylate.

These water-absorbing substances having no function of forming foamingnuclei may be used alone or in combination of two or more of them.

Among them, it is more preferable that the water-absorbing substancehaving no function of forming foaming nuclei be either a compound havinga polyalkylene oxide structure or glycerin. In particular, it ispreferable that the water-absorbing substance having no function offorming foaming nuclei be either polyethylene glycol or glycerin.Polyethylene glycol and glycerin are less toxic substances, and theresultant in-mold expanded molded product can be used in food contactapplications.

Another preferred compound having a polyalkylene oxide structure is acopolymer containing polyalkylene oxide as a block portion, inparticular, a polyolefin-polyether block copolymer (such as a copolymercontaining a polyolefin block and a polyalkylene oxide block). Inparticular, a polyolefin-polyethylene oxide block copolymer whosepolyether block portion has a polyethylene oxide structure is morepreferable. A specific example is a copolymer marketed as Pelestat bySanyo Chemical Industries, Ltd.

Because such a copolymer has a polyolefin block, it is satisfactory incompatibility with polyolefin resin. Further, because it is a solid, itis satisfactory in handling and does not cause defective feeding duringkneading or extrusion. In consequence, there does not occur unevendischarge during extrusion, and resin particles uniform in shape can beproduced by a strand cut method. When such resin particles are expanded,the resulting expanded particles are uniform in cell diameter and lessvariable in expansion ratio. An in-mold expanded molded product obtainedby in-mold expansion molding of such expanded particles is small in gapbetween particles, shrinkage, and distortion to be good in appearance,and tends to be high in rate of fusion and sufficient in dimensionalstability under heating.

Further, other preferable examples of the water-absorbing compoundhaving no function of forming foaming nuclei include bentonite,synthetic hectolite, and synthetic zeolite, etc. In general, aninorganic substance has a function of forming foaming nuclei. However,although these substances are inorganic substances, they areunexpectedly low in function of forming foaming nuclei, and thereforecan be suitably used.

It is preferable that the polyethylene glycol, the polyvalent alcoholhaving a carbon number of not less than 3 to not more than 6 and threeor more hydroxyl groups, and the water-absorbing compound having nofunction of forming foaming nuclei, which have been described above,have a melting point of lower than 150° C. A substance having a meltingpoint of lower than 150° C. is preferable because it is highly likely toexist not as a solid but as a liquid during foaming and therefore iseven lower in function of forming foaming nuclei, thus making it easy tocontrol the cell diameter and the expansion ratio. A substance having amelting point of not lower than 150° C. tends to express the function offorming foaming nuclei and, as a result, may cause degradation infusibility and surface properties such as gaps between particles,shrinkage, and distortion when processed into an in-mold expanded moldedproduct.

Specific examples of such a substance having a melting point of lowerthan 150° C. include polyethylene glycol (which has a melting point of−13° C. when it has an average molecular weight of 300), glycerin (whichhas a melting point of 20° C.), and the aforementioned Pelestat (whichhas a melting point of 135° C., in the case of Pelestat 303), which is acopolymer having a polyolefin block and a polyalkylene oxide block, etc.

In the present invention, the polyethylene glycol, the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups, and the water-absorbing compound havingno function of forming foaming nuclei are added in not less than 0.05parts by weight to not more than 2 parts by weight, not less than 0.05parts by weight to not more than 2 parts by weight, and not less than0.01 part by weight to not more than 5 parts by weight to 100 parts byweight of the polyolefin resin, respectively.

In any case, adjustment of the amount of addition makes it possible toadjust the moisture content and the volatile content and change theexpansion ratio. However, if the amount of addition is less than thosespecified above, the function of water or carbon dioxide to improve theexpansion ratio will become less effective, and the function ofuniforming the cell diameter will also become less effective. On theother hand, if the amount of addition exceeds those specified above,there will be shrinkage in the expanded polyolefin resin particles, orthere will be insufficient dispersion into the polyolefin resin.

It is more preferable that the polyethylene glycol, the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups, and the water-absorbing compound havingno function of forming foaming nuclei be added in not less than 0.05parts by weight to not more than 1 part by weight, even more preferablyin not less than 0.1 part by weight to not more than 0.5 parts byweight.

However, the smaller average molecular weight polyethylene glycol has,the more likely it is to increase the moisture content. Polyethyleneglycol having a great average molecular weight tends to be added in alarger amount for equal moisture content. Therefore, the molecularweight and amount of addition of polyethylene glycol that is used can beselected in balance with a desired expansion ratio, a desired moisturecontent, and desired properties.

Further, when the polyvalent alcohol having a carbon number of not lessthan 3 to not more than 6 and three or more hydroxyl groups is glycerin,it is preferable that glycerin be added in not less than 0.05 parts byweight to not more than 0.5 parts by weight to 100 parts by weight ofthe polyolefin resin.

It should be noted here that the content of the polyvalent alcoholhaving a carbon number of not less than 3 to not more than 6 and threeor more hydroxyl groups in the polyolefin resin particles and theexpanded polyolefin resin particles can be determined by using an HPLCapparatus (e.g., high-performance liquid chromatography: prominencehigh-pressure gradient system; manufactured by Shimadzu Corporation) inwhich an ESLD (evaporate light scattering detector) is used.

It should be noted that the polyethylene glycol, the polyvalent alcoholhaving a carbon number of not less than 3 to not more than 6 and threeor more hydroxyl groups, and the water-absorbing compound having nofunction of forming foaming nuclei are added in amounts weighed with nowater absorbed therein.

The foam nucleating agent that is used in the present invention refersto a substance that facilitates the formation of cell nuclei duringfoaming, example of which include talc, calcium carbonate, silica,kaolin, barium sulfate, calcium hydroxide, aluminum hydroxide, aluminumoxide, titanium oxide, zeolite, aliphatic metal salts such as calciumstearate and barium stearate, melamine, and metal borate, etc. Thesefoam nucleating agents preferably have a sharp grain size distribution,and may be used alone or in combination of two or more of them.

Among them, talc, metal borate, and calcium carbonate are preferable. Inparticular, talc is preferred because use of talc, which is inexpensiveand well-suited to the water-absorbing compound having no function offorming foaming nuclei, improves the dispersibility into the polyolefinresin of the water-absorbing compound having no function of formingfoaming nuclei, and makes it easy to obtain an in-mold expanded moldedproduct uniform in cell diameter.

Although the amount of addition is adjusted appropriately depending onthe foaming agent used, the desired expansion ratio, and the like, it ispreferable that the foam nucleating agent be added in not less than0.005 parts by weight to not more than 2 parts by weight, or morepreferably not less than 0.01 part by weight to not more than 1 part byweight, to 100 parts by weight of the polyolefin resin. If the foamnucleating agent is added in less than 0.005 parts by weight, theexpansion ratio will not be able to be increased, or there will bedegradation in uniformity in cell diameter. If the foam nucleating agentis added in more than 2 parts by weight, the in-mold expanded moldedproduct will be so small in average cell diameter as to be defective.

When the foam nucleating agent is talc, it is preferable that talc beused in not less than 0.02 parts by weight to not more than 0.5 parts byweight to 100 parts by weight of the polyolefin resin, use of such anamount of talc makes it easy to attain the desired average cell diameterso that the in-mold expanded molded product is satisfactory.

It is preferable that the polyolefin resin that is used in the presentinvention be polyolefin resin such as polyethylene resin orpolypropylene resin, or more preferably polypropylene resin.

Examples of the polyethylene resin include high-density polyethylene,medium-density polyethylene, low-density polyethylene, linearlow-density polyethylene, an ethylene homopolymer, and anethylene-α-olefin copolymer, etc. Examples of the α-olefin here includeα-olefins having a carbon number of 3 to 15, etc., and these may be usedalone or in combination of two or more of them.

Examples of the polypropylene resin include a propylene homopolymer, apropylene-α-olefin random copolymer, and a propylene-α-olefin blockcopolymer, etc. Examples of the α-olefin here include α-olefins having acarbon number of 2 and 4 to 15, and these may be used alone or incombination of two or more of them. Further, the aforementionedpropylene homopolymer, propylene-α-olefin random copolymer,propylene-α-olefin block copolymer may be used in combination of two ormore of them.

Among them, in particular, those propylene-ethylene random copolymers,propylene-ethylene-butene-1 random copolymers, and propylene-butene-1random copolymers whose comonomer content other than propylene rangesfrom 1 to 5% by weight exhibit satisfactory expandability, and thereforecan be suitably used. Further, the copolymers have such properties as tobe easily impregnated with carbon dioxide, and therefore are suitable.

Although the polyolefin resin that is used in the present invention isnot particularly limited in melt index, it is preferable that thepolyolefin resin have a melt index of not less than 0.5 g/10 minutes tonot more than 30 g/10 minutes, more preferably not less than 2 g/10minutes to not more than 9 g/10 minutes, or most preferably, especiallywhen the polyolefin resin is polypropylene resin, not less than 4 g/10minutes to not more than 8 g/10 minutes. If the polyolefin resin has amelt index of less than 0.5 g/10 minutes, the resulting expandedparticles are neither high in expansion ratio nor uniform in cell. Onthe other hand, if the polyolefin resin has a melt index of more than 30g/10 minutes, there will be such an improvement in expandability thatthe resulting expanded particles are likely to have a high expansionratio; however, there will breakage of foam cells, with the result thatthe expanded particles become high in open cell ratio and nonuniform incell.

It should be noted that when the polyolefin resin is polypropyleneresin, the melt index of the polyolefin resin in the present inventionis a value measured at a temperature of 230° C. with a load of 2.16 kgin conformity to JIS K7210; and when the polyolefin resin ispolyethylene resin, the melt index of the polyolefin resin in thepresent invention is a value measured at a temperature of 190° C. with aload of 2.16 kg in conformity to JIS K7210.

It is preferable that the polyolefin resin that is used in the presentinvention be polypropylene resin, because it makes it easy to obtainexpanded particles excellent in expandability and moldability andexcellent in mechanical strength and heat resistance when processed intoan in-mold expanded molded product.

It is preferable that the polypropylene resin have a melting point ofnot lower than 130° C. to not higher than 165° C., or more preferablynot lower than 135° C. to not higher than 155° C. If the polypropyleneresin has a melting point of lower than 130° C., it tends to beinsufficient in heat resistance and mechanical strength. On the otherhand, if the polypropylene resin has a melting point of higher than 165°C., it tends to make it difficult to secure fusion between expandedparticles during in-mold expansion molding.

The term “melting point” here means the peak temperature of anendothermic peak on a DSC curve that is obtained by raising thetemperature of not less than 1 mg to not more than 10 mg ofpolypropylene resin from 40° C. to 220° C. at a heating rate of 10°C./min in differential scanning calorimetry, lowering the temperature to40° C. at a heating rate of 10° C./min, and then again raising thetemperature to 220° C. at a heating rate of 10° C./min.

In the present invention, additives other than the polyethylene glycol,the polyvalent alcohol having a carbon number of not less than 3 to notmore than 6 and three or more hydroxyl groups, the water-absorbingsubstance having no function of forming foaming nuclei, and the foamnucleating agent can be added in such amounts as not to impair theeffects of the present invention. Examples of additives include acompatibilizing agent, an antistatic agent, a colorant, a stabilizer, aweatherproofer, and a flame retardant, etc.

In the present invention, the polyethylene glycol, the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups, the water-absorbing compound having nofunction of forming foaming nuclei, and the polyolefin resin, which havebeen described above, are used as the polyolefin resin particles.

The polyolefin resin particles can be obtained by using a well-knownmethod, e.g., by blending (a) polyethylene glycol, (b) the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups, or (c) the water-absorbing compoundhaving no function of forming foaming nuclei and the foam nucleatingagent with the polyolefin resin, melting and kneading the mixture in anextruder, extruding it through a die, cooling it, and then cutting itinto polyolefin resin particles with a cutter. When a substance selectedas the polyethylene glycol, the polyvalent alcohol having a carbonnumber of not less than 3 to not more than 6 and three or more hydroxylgroups, or the water-absorbing compound having no function of formingfoaming nuclei is a substance that takes the form of a liquid or wax(semiliquid) at room temperature (e.g., polyethylene glycol having amolecular weight of not more than 3,000), the polyolefin resin particlesmay be obtained either by using the aforementioned method or bysupplying a fixed quantity of such a substance in the form of a liquidto the molten polyolefin resin either in the input hopper area of theextruder or in the middle of the extruder and kneading the mixture. Whenadded in the form of a liquid, a substance that takes the form of wax atlow temperatures, such as polyethylene glycol having a molecular weightof 1,000 to 3,000 or glycerin, only has to be added after being heatedand melted.

Alternatively, in the case of a substance that tends to evaporate atextrusion temperature, such as polyethylene glycol having a molecularweight of not more than 4,000, it is desirable to set the cylinder anddie area of the extruder to a low temperature of 250° C. or lower sothat the substance evaporates less.

It should be noted that it is possible to prepare a masterbatch of thepolyethylene glycol, the polyvalent alcohol having a carbon number ofnot less than 3 to not more than 6 and three or more hydroxyl groups, orthe water-absorbing compound having no function of forming foamingnuclei and the foam nucleating agent with polyolefin resin, to blend themasterbatch with polyolefin resin so that they are finally added indesired amounts, and to knead and melt the mixture in the extruder togive polyolefin resin particles.

The following describes a process of the present invention for producingexpanded polyolefin resin particles.

Expanded polyolefin resin particles in the present invention areproduced by: dispersing such polyolefin resin particles produced asmentioned above together with an aqueous dispersion medium into a closedvessel; heating the polyolefin resin particles up to or above thesoftening temperature of the polyolefin resin particles; and releasingthe polyolefin resin particles into a zone whose pressure is lower thanan internal pressure of the closed vessel. It should be noted here thatbecause water contained in the aqueous dispersion medium serves as afoaming agent and inorganic gas such as carbon dioxide, nitrogen, or airis injected into the closed vessel at any stage before the release intothe low-pressure zone, it is possible to increase the internal pressureof the closed vessel, to regulate the rate of pressure release duringthe foaming, and to adjust the expansion ratio and the average celldiameter.

In a more preferred aspect of the present invention, i.e., when carbondioxide is further added as a foaming agent, the polypropylene resinparticles, water, and solid carbon dioxide (dry ice) may be poured intothe closed vessel or, after the polyolefin resin particles and water arepoured into the closed vessel, gaseous or liquid carbon dioxide may beintroduced into the closed vessel at any stage before the release intothe low-pressure zone. Alternatively, a combination of these methods canbe employed.

Such concomitant use of water and carbon dioxide as foaming agents makesit easy to increase foaming power, and therefore makes it possible toreduce the amount of addition of the foam nucleating agent in attaininga high expansion ratio, thus giving expanded particles large in averagecell diameter and satisfactory in secondary expandability. Further,carbon dioxide is presumed to become likely to be retainedsimultaneously with water in the polyethylene glycol, the polyvalentalcohol having a carbon number of not less than 3 to not more than 6 andthree or more hydroxyl groups, or the water-absorbing compound having nofunction of forming foaming nuclei. This is preferable because itbecomes possible to form cells uniform in diameter, and it becomeseasier to control the cell diameter and the expansion ratio.

The present invention uses water as a foaming agent, and “use of wateras a foaming agent” can be determined by measuring the moisture contentmentioned later. The moisture content can be measured by another method,e.g., by using a polymer moisture measuring system or Karl Fischermoisture measuring system to measure the expanded particles immediatelyafter the foaming.

It is preferable carbon dioxide that is used concomitantly as a foamingagent in the present invention be added in not less than 0.5 parts byweight to not more than 30 parts by weight, or more preferably not lessthan 2 parts by weight to not more than 20 parts by weight, to 100 partsby weight of the polyolefin resin particles. If carbon dioxide is addedin less than 0.5 parts by weight, the effect of concomitant use withwater as a foaming agent is unlikely to be exerted. On the other hand,if carbon dioxide is added in more than 30 parts by weight, the foamingpower becomes so high that the resulting expanded particles tend to havea higher open cell ratio, e.g., to suffer from cell breakage.

In the present invention, it is essential to use water as a foamingagent, and it is preferable to concomitantly use water and carbondioxide as foaming agents. However, it is also possible tosupplementarily use other physical foaming agents as needed, forexample, to use saturated hydrocarbons having a carbon number of 3 to 5,dimethyl ether, alcohols such as methanol and ethanol, which have aboiling point lower than the expandable temperature, and inorganicfoaming agents such as air and nitrogen.

It should be noted that it is the water contained in the aqueousdispersion medium in the closed vessel that is used as a foaming agent.

Further, the other physical foaming agents may be introduced before orduring the heating after dispersing the polyolefin resin particlestogether with the aqueous dispersion medium into the closed vessel, ormay be introduced after the heating or immediately before the foaming.Alternatively, the other physical foaming agents may be introducedduring the foaming so that the internal pressure of the closed vesselwill not decrease during the foaming. Furthermore, the other physicalfoaming agents may be introduced in several batches.

From the point of view of sufficiently impregnating the polyolefin resinparticles with the foaming agents for higher foaming power, it ispreferable that the foaming agents be introduced before the heating.From the point of view of inhibiting the resulting expanded polyolefinresin particles from varying in expansion ratio, it is preferable thatthe foaming agents be also introduced during the foaming.

A more specific example of a process for producing expanded polyolefinresin particles includes the following steps.

After polyolefin resin particles, an aqueous dispersion medium and, asneeded, a dispersant and the like are fed into a closed vessel and avacuum is created, as needed, in the closed vessel, carbon dioxide isintroduced at approximately not lower than 1 MPa to not higher than 2MPa (gage pressure) and the temperature is raised up to or above thesoftening temperature of the polyolefin resin. The heating causes theinternal pressure of the closed vessel to increase to approximately notlower than approximately 1.5 MPa to not higher than 5 MPa (gagepressure). After carbon dioxide is further added at a temperature closeto the foaming temperature so that a desired foaming pressure isattained and a temperature adjustment is further made, the release intoa zone whose pressure is lower than the internal pressure of the closedvessel is carried out. In this way, the expanded polyolefin resinparticles are obtained.

Alternatively, after polyolefin resin particles, an aqueous dispersionmedium and, as needed, a dispersant and the like are fed into a closedvessel and a vacuum is created, as needed, in the closed vessel, carbondioxide may be introduced while raising the temperature up to or abovethe softening temperature of the polyolefin resin.

Alternatively, the expanded polyolefin resin particles are obtained byfeeding polyolefin resin particles, an aqueous dispersion medium and, asneeded, a dispersant and the like into a closed vessel, raising thetemperature up to a temperature close to the foaming temperature,introducing air, nitrogen, or the like, attaining the foamingtemperature, and carrying out the release into a zone whose pressure islower than the internal pressure of the closed vessel.

There are no particular limitations on the closed vessel into which thepolyolefin resin particles are dispersed, as long as it can withstandits internal pressure and temperature during production of the expandedparticles, and examples of the closed vessel include autoclavepressure-resistant containers.

It is preferable that the aqueous dispersion medium be water. Theaqueous dispersion medium may also be a dispersion medium obtained byadding methanol, ethanol, ethylene glycol, glycerin, or the like towater.

It is preferable to use a dispersant to prevent the polypropylene resinparticles from being closely united with each other in the aqueousdispersion medium. Examples of the dispersant include inorganicdispersants such as tertiary calcium phosphate, tertiary magnesiumphosphate, basic magnesium carbonate, calcium carbonate, barium sulfate,kaolin, talc, and clay.

Further, it is preferable to use a dispersion auxiliary agent togetherwith the dispersant. Examples of the dispersion auxiliary agent include:anionic surfactants of the carboxylate type such as N-acylamino-acidsalt, alkyl ether carboxylate, and acyl peptide; anionic surfactants ofthe sulfonate type such as alkyl sulfonate, alkyl benzene sulfonate,alkyl naphthalene sulfonate, and sulfosuccinate; anionic surfactants ofthe sulfuric ester type such as sulfated oil, alkyl sulfate, alkyl ethersulfate, and alkyl amide sulfate; and anionic surfactants of thephosphoric ester type such as alkyl phosphate, polyoxyethylenephosphate, and alkyl aryl ether phosphate. It is also possible to use:polymer surfactants of the polycarboxylic acid type such as a salt of amaleic acid copolymer and polyacrylate; and polyanionic polymersurfactants such as polyethylene sulfonate and a salt of a naphthalenesulfonate formalin condensate.

It is preferable that the dispersion auxiliary agent be an anionicsurfactant of the sulfonate type, more preferably one type selected fromalkyl sulfonate and alkyl benzene sulfonate or a mixture of two or moretypes; even more preferably alkyl sulfonate, or especially preferablyalkyl sulfonate having a C10 to C18 straight carbon chain as ahydrophobic group, which can reduce adhesion of the dispersant to theexpanded polyolefin resin particles.

Among them, it is preferable to concomitantly use, as the dispersant,one or more type selected from tertiary calcium phosphate, tertiarymagnesium phosphate, barium sulfate, and kaolin, and to concomitantlyuse alkyl sulfonate as the dispersion auxiliary agent.

The amounts of the dispersant and the dispersion auxiliary agent thatare used vary according to their types and the type and amount of thepolyolefin resin that is used. Usually, it is preferable to blend thedispersant and the dispersion auxiliary agent in not less than 0.2 partsby weight to not more than 3 parts by weight and in not less than 0.001part by weight to not more than 0.1 part by weight, respectively, to 100parts by weight of the aqueous dispersion medium. Further, it ispreferable to use the polyolefin resin particles in not less than 20parts by weight to not more than 100 parts by weight to 100 parts byweight of the aqueous dispersion medium so that the dispersibility ofthe polyolefin resin particles in the aqueous dispersion medium issatisfactory.

It is preferable that the expanded polyolefin resin particles thusobtained of the present invention contain not less than 0.05% by weightto not more than 2% by weight of the polyethylene glycol or thepolyvalent alcohol having a carbon number of not less than 3 to not morethan 6 and three or more hydroxyl groups, or not less than 0.01% byweight to not more than 5% by weight of the water-absorbing compoundhaving no function of forming foaming nuclei.

It is preferable that the expanded polyolefin resin particles thusproduced of the present invention have a melt index of not less than 1g/10 minutes to not more than 12 g/10 minutes, more preferably not lessthan 3 g/10 minutes to not more than 11 g/10 minutes, or most preferablynot less than 4 g/10 minutes to not more than 10 g/10 minutes. If theexpanded polyolefin resin particles have a melt index of less than 1g/10 minutes, degradation in secondary expandability leads todegradation in moldability during in-mold expansion molding. On theother hand, if the expanded polyolefin resin particles have a melt indexof more than 12 g/10 minutes, the expanded particles become higher inopen cell ratio, e.g., to suffer from cell breakage.

It should be noted here that when the expanded polyolefin resinparticles are expanded polypropylene resin particles, the melt index ofthe expanded polyolefin resin particles in the present invention is avalue measured at a temperature of 230° C. with a load of 2.16 kg inconformity to JIS K7210; and when the expanded polyolefin resinparticles are expanded polyethylene resin particles, the melt index ofthe expanded polyolefin resin particles in the present invention is avalue measured at a temperature of 190° C. with a load of 2.16 kg inconformity to JIS K7210. In either case, the value is measured afterdefoaming the expanded polyolefin resin particles by pressing andturning them into a sheet at 190° C. under 10 MPa and then crushing thesheet into pieces of suitable size.

According to the process of the present invention for producing expandedpolyolefin resin particles, the melt index of expanded polyethyleneresin particles tends to be smaller than the melt index of the baseresin, polyethylene resin, and the difference is less than approximately3 g/10 minutes. Selection of a melt index of the polyethylene resin withthis point in view makes it possible to obtain the expanded polyethyleneresin particles of the present invention.

On the other hand, the melt index of expanded polypropylene resinparticles tends to be larger than the melt index of the base resin,polypropylene resin, and the difference is less than approximately 3g/10 minutes. Selection of a melt index of the polypropylene resin withthis point in view makes it possible to obtain the expandedpolypropylene resin particles of the present invention.

There are no particular limitations on the expansion ratio of theexpanded polyolefin resin particles, and it is possible to produce theexpanded particles having an expansion ratio of more than 1 time to lessthan 3 times. However, it is preferable that the expansion ratio be notless than 3 times, more preferably not less than 8 times, or even morepreferably not less than 10 times. Further, it is preferable that theupper limit of the expansion ratio be not more than 50 times, morepreferably not more than 45 times, or even more preferably not more than25 times.

If the expansion ratio is less than 3 times, the merit of being light inweight cannot be attained, and the resulting in-mold expanded moldedproduct tends to be insufficient in flexibility and bufferingproperties. If the expansion ratio is more than 50 times, the resultingin-mold expanded molded product tends to be insufficient in dimensionalaccuracy, mechanical strength, heat resistance, and the like.

If the expansion ratio is not less than 3 times, the cell diameter andthe expansion ratio tend to be able to be adjusted satisfactorily byconcomitant use of the foam nucleating agent with the polyethyleneglycol, the polyvalent alcohol having a carbon number of not less than 3to not more than 6 and three or more hydroxyl groups, or thewater-absorbing compound having no function of forming foaming nuclei.On the other hand, if the expansion ratio is not more than 50 times, anin-mold expanded molded product obtained by in-mold expansion moldingtends to be sufficient in dimensional accuracy, mechanical strength,heat resistance, and the like, without breakage of cells in the expandedpolypropylene resin particles.

In order to obtain expanded polyolefin resin particles having anexpansion ratio of not less than 20 times, it is possible to attain anexpansion ratio of not less than 20 times in the step of releasing thepolyolefin resin particles into the zone whose pressure is lower thanthe internal pressure of the closed vessel (this step being referred tosometimes as “first-stage foaming”). However, it is preferable toproduce expanded polyolefin resin particles with an expansion ratio ofless than 20 times by first-stage foaming, to pressurize the expandedpolyolefin resin particles with an inorganic gas such as air in a closedvessel to impart internal pressure, and to increase the expansion ratioto 20 times or higher by further expanding the expanded polyolefin resinparticles (referred to sometimes as “second-stage foaming”) by steamheating.

It is preferable that the expanded polyolefin resin particles of thepresent invention have an average cell diameter of not less than 50 μmto not more than 800 μm, more preferably not less than 130 μm to notmore than 500 μm, or most preferably not less than 210 μm to not morethan 350 μm. If the average cell diameter is less than 50 μm, there mayoccur such problems as distortion in the shape of the resulting in-moldexpanded molded product and generation of wrinkles on a surface of theproduct. If the average cell diameter is more than 800 μm, there may bedegradation in buffering properties of the resulting in-mold expandedmolded product.

It is preferable that the expanded polyolefin resin particles of thepresent invention have an open cell ratio of not less than 0% to notmore than 12%, more preferably not less than 0% to not more than 8%, oreven more preferably not less than 0% to not more than 5%. If the opencell ratio is more than 12%, there occurs deterioration in theexpandability of the expanded polyolefin resin particles during in-moldsteam heating when the expanded polyolefin resin particles are subjectedto in-mold expansion molding, and the resultant in-mold expanded moldedproduct has large holes and therefore tends to shrink.

It is preferable that the expanded polyolefin resin particles of thepresent invention have a moisture content of not lower than 0.1% to nothigher than 10%, more preferably not lower than 0.7% to not higher than8%, or even more preferably not lower than 1% to not higher than 5%. Ifthe moisture content is lower than 0.1%, the resulting expandedpolyolefin resin particles may only have a low expansion ratio. If themoisture content is higher than 10%, the internal pressure of theexpanded polyolefin resin particles after foaming is so low that theyare likely to shrink and may remain shrunk even when cured in an ovenafter foaming.

It is preferable that the expanded polyolefin resin particles of thepresent invention have two melting peaks on a DSC curve that is obtainedby differential scanning calorimetry as shown in FIG. 1.

Those expanded particles which have two melting peaks tend to have suchgood in-mold expansion moldability as to give an in-mold expanded moldedproduct satisfactory in mechanical strength and heat resistance.

It should be noted here that the DSC curve that is obtained bydifferential scanning calorimetry of the expanded polyolefin resinparticles means a DSC curve that is obtained by raising the temperatureof not less than 1 mg to not more than 10 mg of expanded particles from40° C. to 220° C. at a heating rate of 10° C./min in differentialscanning calorimetry.

Such expanded polyolefin resin particles having two melting peaks areeasily obtained by setting the temperature in the closed vessel duringfoaming to an appropriate value. That is, in the case of the presentinvention, the temperature in the closed vessel is usually not lowerthan the softening temperature of the polyolefin resin, which serves asa base material, preferably not lower than the melting point, morepreferably not lower than a temperature of 5° C. plus the melting pointto lower than the temperature at which the melting ends, even morepreferably not higher than a temperature of 2° C. minus the temperatureat which the melting ends. In such a case, expanded polyolefin resinparticles having two melting peaks tend to be obtained.

It should be noted that the term “temperature at which the melting ends”means a temperature at which the base of a melting peak of a DSC curvethat is obtained by raising the temperature of not less than 1 mg to notmore than 10 mg of polyolefin resin from 40° C. to 220° C. at a heatingrate of 10° C./min in differential scanning calorimetry, lowering thetemperature to 40° C. at a heating rate of 10° C./min, and then againraising the temperature to 220° C. at a heating rate of 10° C./minreturns to the position of the baseline on the high-temperature side.

Further, it is preferable that the quantity of heat at the endothermicpeak on the higher-temperature side among the two melting peaks (such aquantity of heat being hereinafter denoted sometimes by “Qh”) be notless than 5 J/g to not more than 30 J/g, or more preferably not lessthan 7 J/g to not more than 20 J/g. If the quantity of heat Qh is lessthan 5 J/g, the expanded polyolefin resin particles tend to have ahigher open cell ratio. If the quantity of heat Qh is more than 30 J/g,there tends to be degradation in fusibility with which an in-moldexpanded molded product is obtained. See FIG. 1 for the quantity of heatQh on the high-temperature side. A is the point at which the smallestquantity of heat absorption is reached between the two melting peaks ofthe DSC curve. Qh is the quantity of heat at the melting peak on thehigher-temperature side, which is the right one of the two areas (shadedin FIG. 1) surrounded by the DSC curve and tangents to the DSC curvedrawn from the point A, and Ql is the quantity of heat at the meltingpeak on the lower-temperature side, which is the left one of the twoshaded areas.

It is preferable that the expanded polyolefin resin particles that areobtained by the present invention have a cell diameter variation of lessthan 0.4, more preferably not more than 0.3, or even more preferably notmore than 0.2. If the cell diameter variation is not less than 0.4, theresulting in-mold expanded molded product deteriorates in surfaceproperties and therefore tends to show noticeable wrinkles and gaps suchas dents and holes between expanded particles (between particles).

Such a cell diameter variation can be attained by using the polyethyleneglycol, the polyvalent alcohol having a carbon number of not less than 3to not more than 6 and three or more hydroxyl groups, or thewater-absorbing compound having no function of forming foaming nucleiand the foam nucleating agent in the present invention or, morepreferably can be easily attained by concomitantly using carbon dioxideas a foaming agent. The reason for this is uncertain, but there presumedto be some effect of the interaction of carbon dioxide with thepolyethylene glycol, the polyvalent alcohol having a carbon number ofnot less than 3 to not more than 6 and three or more hydroxyl groups, orthe water-absorbing compound having no function of forming foamingnuclei.

It is preferable that the expanded polyolefin resin particles of thepresent invention have a volatile content of not lower than 0.1% byweight to not higher than 10% by weight, or more preferably not lowerthan 1% by weight to not higher than 8% by weight. If the volatilecontent is lower than 0.1% by weight, the desired expansion ratiomentioned later cannot be attained. If the volatile content is higherthan 10% by weight, the resulting expanded polyolefin resin particlesshrink and therefore tend to have wrinkles generated on their surfaces.

Such a volatile content is believed to be composed mainly of watercontained in the expanded polyolefin resin particles or carbon dioxide.

For example, in terms of improving the expansion ratio or from the pointof view of in-mold expansion mold ability, a preferred aspect of thevolatile content and average cell diameter of expanded polyolefin resinparticles produced by the process of the present invention for producingexpanded polyolefin resin particles is as follows:

The volatile content and average cell diameter of expanded polyolefinresin particles (referred to as “expanded polyolefin resin particles(P)”) that are obtained by a production process of the present inventionand the volatile content and average cell diameter of expandedpolyolefin resin particles (referred to as “expanded polyolefin resinparticles (Q)”) produced under the same conditions except that thewater-absorbing substance having no function of forming foaming nucleiis not contained satisfy the following formulas (E1) and (E2): (E1):Volatile Content of Expanded Polyolefin Resin Particles (P)≧VolatileContent of Expanded Polyolefin Resin Particles (Q)×1.1; and (E2) AverageCell Diameter of Expanded Polyolefin Resin Particles (P)≧Average CellDiameter of Expanded Polyolefin Resin Particles (Q)×0.7. If the volatilecontent of the expanded polyolefin resin particles (P) is less than thevolatile content of expanded polyolefin resin particles (Q)×1.1, theretends to be degradation in foaming power during foaming, with the resultthat the desired expansion ratio may not be attained.

Further, if the average cell diameter of the expanded polyolefin resinparticles (P) is less than the average cell diameter of the expandedpolyolefin resin particles (Q)×0.7, the decrease in the average celldiameter may lead to degradation in fusibility during in-mold expansionmolding. It should be noted that the average cell diameter in this caseis an average cell diameter L(av) measured according to a methodmentioned later.

EXAMPLES

In the following, the present invention is described more specificallywith reference to Examples and Comparative Examples; however, thepresent invention is not limited solely to these Examples.

It should be noted that evaluations in Examples, Comparative Examples,and References were carried out according to the following methods.

(Expansion Ratio)

Approximately not less than 3 g to not more than 10 g of expandedparticles were taken to be dried at 60° C. for six hours, and then theweight w was measured. After that, the expanded particles were submergedin water contained in a measuring cylinder, and then the absolutespecific gravity ρb=w/v of the expanded particles was calculated bymeasuring the volume v (cm³) according to the surface elevation, and theexpansion ratio K=ρr/ρb was calculated from the ratio of the density ρr(=0.9 g/cm³) of the raw material composition to the absolute specificgravity ρb.

(Open Cell Ratio)

The open cell ratio was calculated by: calculating the closed-cellvolume of the resultant expanded particles using an air comparisonpycnometer (Type 1000; manufactured by Tokyoscience Co., Ltd.);calculating the percentage of closed cells (%) by dividing theclosed-cell volume by the apparent volume separately calculated bysubmergence; and subtracting the percentage of closed cells from 100.

(Moisture Content)

Expanded particles immediately after foaming with use of only water as afoaming agent were used. The expanded particles were dehydrated by usingthe flow of air to blow away moisture from the surfaces of theparticles. After that, the weight (W1) of the expanded particles wasmeasured. Furthermore, the weight (W2) of the expanded particles asdried for twelve hours in an oven at 80° C. was measured. The moisturecontent was computed as:

Moisture Content(%)=(W1−W2)/W2×100.

In the case of concomitant use of a foaming agent other than water, themoisture content of expanded particles just obtained by separate foamingat the same foaming temperature and the same foaming pressure as in thecase of use of only water as a foaming agent was computed through theabove-mentioned operation.

(Volatile Content)

Expanded particles immediately after foaming were used.

The expanded particles were dehydrated by using the flow of air to blowaway moisture from the surfaces of the particles. After that, the weight(w1) of the expanded particles was measured. Furthermore, the weight(w2) of the expanded particles as dried for twelve hours in an oven at80° C. was measured. The volatile content was computed as:

Volatile Content(%)=(w1−w2)/w2×100.

(Average Cell Diameter d)

In the present invention, two types of average cell diameter weremeasured: one was the average cell diameter d, and the other was theaverage cell diameter L(av) mentioned later. However, the average celldiameter d and the average cell diameter L(av) were roughly equal toeach other. The average cell diameter d was determined as below.

Each expanded particle was cut substantially in the middle withsufficient care to prevent the destruction of a cell membrane, and across-section of each expanded particle was magnified and observed witha microscope. Let it be assumed that the x direction is a givendirection of that portion (H) of each expanded particle excluding thesurface part whose length is five percent of the diameter of theexpanded particle and the y direction is a direction orthogonal to the xdirection. Then, the Feret's diameters of each cell along the x and ydirections were measured as dx and dy, respectively, and the diameter diof that one cell was calculated as di=(dx+dy)/(2×0.785). Suchmeasurements were performed on forty or more consecutively adjacentcells in such a way that there was no radial bias in the portion (H),and the average of the measurements was given as: Average Cell Diameterd=Σ(di)/n, where n is the number of cells measured.

(Uniformity U of Cells)

Uniformity of cells was measured by using both the uniformity U of cellsand the cell diameter variation S as indices. The uniformity U of cellswas determined as below.

The standard deviation σ of variations in the diameter of the forty ormore cells as measured in determining the average cell diameter d wascomputed, and the uniformity u of cells in a single expanded particlewas computed as u=σ/d×100.

Such measurements were performed on three or more expanded particles,and the average of the measurements was obtained as the uniformity U ofcells, which was evaluated as follows:

Very good: U is 30 or less

Good: U is more than 30 and 35 or less

Poor: U is greater than 35

(Average Cell Diameter L(av))

Twenty randomly chosen expanded particles were cut substantially in themiddle, and each of the cross-sections thus exposed was observed with amicroscope. It should be noted here that the center O is the point oforthogonal intersection between an X axis and a Y axis substantially inthe center of the cross-section, A and A′ are the points at which the Xaxis intersects with the edge of the cross-section, and B and B′ are thepoints at which the Y axis intersects with the edge of thecross-section.

Next, the cell diameter L(OA) was calculated by: counting the number ofcell walls crossed by the segment OA, obtaining a quotient by dividingthe length of the segment OA by the number of cell walls, and furtherdividing the quotient by 0.616. That is, the cell diameter L(OA) wascalculated from Eq. (1) as follows:

[Math. 1]

L(OA)=Length of Segment OA/Number of Cell Walls/0.616  Eq. (1).

The cell diameters L(OA′), L(OB), and L(OB′) were calculated in the sameway based on the segments OA′, OB, and OB′, respectively. It should benoted that a cell wall on which the center O was located was counted.

The average cell diameter L(av) was obtained by computing the averageL′(av) of the four cell diameters L(OA), L(OA′), L(OB), and L(OB′) andfurther averaging the respective averages L′(av) of the twenty expandedpolyolefin resin particles.

(Cell Diameter Variation S)

In the measurement of the average cell diameter L(av), the cell diametervariation (S′) of each single expanded particle was computed from Eq.(2) as follows:

[Math. 2]

S′=Σ{(L(i)−L′(av))/L′(av)}²  Eq. (2),

where i=OA, OA′, OB, OB′.

The cell diameter variation S was obtained by averaging the respectivecell diameter variations S′ of the twenty expanded particles.

(Shrinkage and Wrinkles on Expanded Particles)

Evaluations were carried out as follows:

Good: Expanded particles have no wrinkle on their surfaces and thereforeare good in appearance

Poor: Expanded particles are shrunken to have many wrinkles on theirsurfaces

(Second-Stage Expandability)

Expanded particles obtained by two-stage foaming were observed with eyesand evaluated as follows:

Good: There are no expanded particles agglomerated to each other

Average: There are a few expanded particles sticking to each other

Poor: A high vapor pressure is required; therefore, there are manyexpanded particles agglomerated to each other

(Surface Properties of Molded Products)

The surface properties of each molded product obtained by leaving it atrest for two hour at 23° C. after in-mold expansion molding, curing itfor six hour at 65° C., and leaving it in a room for four hours at 23°C. were evaluated by the following criteria:

Very good: No wrinkles, no gaps between particles (no dents or holesbetween expanded particles), or no surface sinks; good in appearance

Good: Slight wrinkles, slight gaps between particles; but no surfacesinks (with no practical problem)

Average: Slight wrinkles, slight gaps between particles, and slightsurface sinks (with no practical problem)

Poor: Noticeable wrinkles, gaps between particles, and surface sinks;defective in appearance

(Rate of Dimensional Shrinkage of Molded Products)

The longitudinal dimensions of each molded product obtained by leavingit at rest for two hour at 23° C. after in-mold expansion molding,curing it for six hour at 65° C., and leaving it in a room for fourhours at 23° C. were measured. The ratio of difference of the dimensionsof the in-mold expanded molded product to those of the correspondingmold was obtained as a rate of dimensional shrinkage with respect to themold, which was evaluated by the criteria below. It should be noted thatsuch a mold was used for molding that the design dimensions of a moldedproduct were 400 mm×300 mm×20 mm.

Very good: The rate of dimensional shrinkage with respect to the mold is4% or lower

Good: The rate of dimensional shrinkage with respect to the mold ishigher than 4% and 7% or lower

Average: The rate of dimensional shrinkage with respect to the mold ishigher than 7% and 9% or lower

Poor: The rate of dimensional shrinkage with respect to the mold ishigher than 9%

(Rate of Fusion of Molded Products)

Each molded product was cracked with a knife approximately 5 mm deep onthe surface. The in-mold expanded molded product was split along thecrack. The fracture surface was observed. The ratio of the number ofbroken particles to the total number of particles observed was obtained.Evaluations were based on the following criteria:

Very good: The rate of fusion is 80% or higher

Good: The rate of fusion is 65% or higher and lower than 80%

Average: The rate of fusion is 50% or higher and lower than 65%

Poor: The rate of fusion is lower than 50%

Example 1

Polyethylene glycol (having an average molecular weight of 300;manufactured by Lion Corporation) was pre-blended in 0.5 parts by weightto 100 parts by weight of polypropylene resin A (propylene-ethylenerandom copolymer: ethylene content of 3.0%, MI=6 g/10 minutes, meltingpoint of 143° C.). In addition, talc (manufactured as Talcan Powder PK-Sby Hayashi-Kasei Co., Ltd.) was blended as a foam nucleating agent in0.05 parts by weight. The mixture was supplied to a single screwextruder 50 in diameter, melted and kneaded, extruded through acylindrical die 1.8 mm in diameter, cooled with water, and then cut witha cutter to give cylinder-shaped polypropylene resin particles (1.2mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thepolypropylene resin particles thus obtained were fed together with 200parts by weight of pure water, 1.0 part by weight of tertiary calciumphosphate, and 0.05 parts by weight of soda dodecylbenzenesulfonate.After deairing, and while stirring, 6 parts by weight of carbon dioxidewere put into the closed vessel, which was then heated to 148° C. Theinternal pressure of the pressure-resistant closed vessel at this pointin time was 3 MPa (gage pressure). Shortly after that, the waterdispersion (the resin particles and the aqueous dispersion medium) wasreleased into a zone under atmospheric pressure through an orifice 4 mmin diameter by opening a valve installed in the lower part of the closedvessel, thus giving expanded particles (first-stage expanded particles).During the release, the internal pressure of the vessel was retained bycarbon dioxide so as not to decrease.

The first-stage expanded particles thus obtained showed two meltingpoints of 138° C. and 157° C. in differential scanning calorimetry. As aresult of the measurement of the expansion ratio, the open cell ratio,and the average cell diameter, the first-stage expanded particles werefound to have an expansion ratio of 19 times and an open cell ratio of0.6%, to be excellent in uniformity of cells, and to have an averagecell diameter d of 340 μm. The moisture content was measured by waterfoaming with the internal temperature of the closed vessel set at 148°C. as above, and was found to be 3.3%.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by drying them for six hour at 60° C., setting theinternal pressure at approximately 0.4 MPa (absolute pressure) throughimpregnating them with pressurized air in the closed vessel, andbringing them into contact with vapor of approximately 0.08 MPa (gagepressure), thus giving second-stage expanded particles having anexpansion ratio of 30 times. The second-stage expanded particles showedtwo melting points in differential scanning calorimetry, had an opencell ratio of 1.3%, and were excellent in uniformity of cells an averagecell diameter d of 435 μm. As a result of the microscopic observation ofthe surfaces of the expanded particles subjected to the second-stagefoaming, the expanded particles were found to be uniform in diameter ofcells in the surfaces, to have smooth surfaces, and to be small innumber of thin parts of the surface membranes thereof. Next, theexpanded particles subjected to the second-stage foaming were subjectedto in-mold expansion molding after setting the internal air pressure atapproximately 0.19 MPa (absolute pressure) through again pressuring themwith air in the closed vessel. The in-mold expanded molded product thusobtained had a surface excellent in smoothness and free of wrinkles, wassmall in dimensional shrinkage, had less distortion, was excellent infusion between particles, and therefore was good in appearance. Theresults are shown in Table 1.

TABLE 1 Examples 1 2 3 4 5 6 7 Polypropylene resin A (Parts by 100 100100 100 100 100 100 weight) Linear low-density polyethylene resin (Partsby weight) Polyethylene glycol (Parts by 0.5 0.2 0.1 0.05 0.5 (avg.molecular wt.: 300) weight) Polyethylene glycol (Parts by 0.5 (avg.molecular wt.: 600) weight) Polyethylene glycol (Parts by 1.0 (avg.molecular wt.: 6,000) weight) Cross-linked polyalkylene oxide (Parts byweight) Sodium polyacrylate (Parts by weight) Carboxymethylcellulosesodium (Parts by weight) Zeolite (Parts by weight) Polypropylene glycol(Parts by (avg. molecular wt.: 2,000) weight) Talc (Parts by 0.05 0.10.05 0.1 0.05 0.1 0.1 weight) Percentage of moisture content (%) 3.3 2.01.3 2.2 0.7 3.3 3.0 Amount of CO₂ (Parts by 6 6 6 6 3 0 0 weight) First-Expansion ratio (Times) 19 15 11 12 6 12 10 stage Average cell diameterd (μm) 340 270 275 260 200 235 225 expanded Uniformity of cells VeryGood Very Good Very Good Good Good Good Good particles Second-stageexpandability Good Good Good Good Good Good Good Molded Surfaceproperties Very good Very good Very good Good Very good Good GoodProduct Rate of dimensional shrinkage Very good Very good Very good GoodVery good Good Good Rate of fusion Very good Very good Very good GoodVery good Very good Very good Examples Comp. Examples 8 9 10 11 12 1 2Polypropylene resin A (Parts by 100 100 100 100 100 100 weight) Linearlow-density polyethylene resin (Parts by 100 weight) Polyethylene glycol(Parts by 0.5 (avg. molecular wt.: 300) weight) Polyethylene glycol(Parts by (avg. molecular wt.: 600) weight) Polyethylene glycol (Partsby (avg. molecular wt.: 6,000) weight) Cross-linked polyalkylene oxide(Parts by 1.0 weight) Sodium polyacrylate (Parts by 0.5 weight)Carboxymethylcellulose sodium (Parts by 0.3 weight) Zeolite (Parts by1.0 weight) Polypropylene glycol (Parts by 0.2 (avg. molecular wt.:2,000) weight) Talc (Parts by 0.05 0.05 0.1 0.1 0.1 0.05 0 weight)Percentage of moisture content (%) 2.3 2.8 3.5 0.7 2.4 0.4 4.0 Amount ofCO₂ (Parts by 6 6 6 6 12 6 6 weight) First- Expansion ratio (Times) 1117 18 9 5 6 13 stage Average cell diameter d (μm) 320 380 300 160 160150 225 expanded Uniformity of cells Good Average Average Good Good GoodPoor particles Second-stage expandability Good Good Good Average GoodPoor Average Molded Surface properties Good Average Average Average GoodPoor Poor Product Rate of dimensional shrinkage Average Average AverageAverage Good Poor Poor Rate of fusion Average Average Very good AverageVery good Very good Average

Example 2

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 1, except that the additive,polyethylene glycol (having an average molecular weight of 300), andtalc were used in 0.2 parts by weight and 0.1 part by weight,respectively. The first-stage expanded particles showed two meltingpoints, had an expansion ratio of 15 times and an open cell ratio of0.7%, were excellent in uniformity of cells, and had an average celldiameter d of 270 μm. The moisture content was 2.0%. Next, second-stageexpanded particles having an expansion ratio of 30 times were obtainedin the same way as in Example 1. The second-stage expanded particlesshowed two melting points in differential scanning calorimetry, had anopen cell ratio of 0.8%, and were excellent in uniformity of cells withan average cell diameter d of 375 μm. As a result of the evaluation ofin-mold expansion molding, the in-mold expanded molded product thusobtained was found to have a surface excellent in smoothness and free ofwrinkles, to be small in dimensional shrinkage, to have less distortion,to be excellent in fusion between particles, and to be therefore good inappearance. The results are shown in Table 1.

Example 3

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 1, except that the additive,polyethylene glycol (having an average molecular weight of 300), wasused in 0.1 part by weight. The first-stage expanded particles obtainedby the first-stage foaming showed two melting points, had an expansionratio of 11 times and an open cell ratio of 0.7%, were excellent inuniformity of cells, and had an average cell diameter d of 275 μm. Themoisture content was 1.3%. Next, second-stage expanded particles havingan expansion ratio of 30 times were obtained in the same way as inExample 1. The second-stage expanded particles showed two melting pointsin differential scanning calorimetry, had an open cell ratio of 0.8%,and were excellent in uniformity of cells with an average cell diameterd of 420 μm. As a result of the evaluation of in-mold expansion molding,the in-mold expanded molded product thus obtained was found to have asurface excellent in smoothness and free of wrinkles, to be small indimensional shrinkage, to have less distortion, to be excellent infusion between particles, and to be therefore good in appearance. Theresults are shown in Table 1.

Example 4

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 1, except that the additive,polyethylene glycol (having an average molecular weight of 6,000), andtalc were used in 1.0 part by weight and 0.1 part by weight,respectively. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points and had an expansion ratioof 12 times, an open cell ratio of 1.3%, and an average cell diameter dof 260 μm. The cells were slightly inferior in uniformity to those ofExamples 1 to 3, but were substantially uniform. Further, the moisturecontent was 2.2%. Next, second-stage expanded particles having anexpansion ratio of 30 times were obtained in the same way as inExample 1. The second-stage expanded particles showed two melting pointsin differential scanning calorimetry, had an open cell ratio of 2.0%,and were satisfactory in uniformity of cells with an average celldiameter d of 390 μm. As a result of the evaluation of in-mold expansionmolding, the in-mold expanded molded product thus obtained was found tohave a surface excellent in smoothness, albeit with slight wrinkles andgaps between particles, to be comparatively small in dimensionalshrinkage, to have less distortion, and to be therefore good inappearance. As for the fusion between particles, there were slightlymore unfused particles than in Examples 1 to 3. The results are shown inTable 1.

Example 5

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 1, except that the additive,polyethylene glycol (having an average molecular weight of 300), andcarbon dioxide were used in 0.05 parts by weight and 3 parts by weight,respectively. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points and had an expansion ratioof 6 times, an open cell ratio of 0.7%, and an average cell diameter dof 200 μm. The cells were slightly inferior in uniformity to those ofExamples 1 to 3, but were substantially uniform. Further, the moisturecontent was 0.7%. Next, second-stage expanded particles having anexpansion ratio of 30 times were obtained in the same way as inExample 1. The second-stage expanded particles showed two melting pointsin differential scanning calorimetry, had an open cell ratio of 0.8%,and were satisfactory in uniformity of cells with an average celldiameter d of 330 μm. As a result of the evaluation of in-mold expansionmolding, the in-mold expanded molded product thus obtained was found tohave a surface excellent in smoothness and free of wrinkles, to be smallin dimensional shrinkage, to have less distortion, to be excellent infusion between particles, and to be therefore good in appearance. Theresults are shown in Table 1.

Example 6

The additive, polyethylene glycol (having an average molecular weight of300), was used in 0.5 parts by weight, and talc was used in 0.1 part byweight. The foaming agent, carbon dioxide, was not used, but nitrogengas was introduced instead. The temperature was raised to 151° C.Further, during the release of the water dispersion (the resin particlesand the aqueous dispersion medium) into a zone under atmosphericpressure through an orifice 4 mm in diameter, the internal pressure ofthe vessel was retained by nitrogen gas so as not to decrease. As forthe rest, first-stage foaming, second-stage foaming, and the evaluationof in-mold expansion molding were carried out in the same way as inExample 1. The internal pressure of the closed vessel during thefirst-stage foaming was 3.0 MPa (gage pressure). The first-stageexpanded particles obtained by the first-stage foaming showed twomelting points and had an expansion ratio of 12 times, an open cellratio of 1.1%, and an average cell diameter d of 235 μm. The cells wereslightly inferior in uniformity to those of Examples 1 to 3, but weresubstantially uniform. The moisture content was 3.3%. Next, second-stageexpanded particles having an expansion ratio of 30 times were obtainedin the same way as in Example 1. The second-stage expanded particlesshowed two melting points in differential scanning calorimetry, had anopen cell ratio of 2.3%, and were satisfactory in uniformity of cellswith an average cell diameter d of 355 μm. As a result of the evaluationof in-mold expansion molding, the in-mold expanded molded product thusobtained was found to have a surface excellent in smoothness, albeitwith slight wrinkles and gaps between particles, to be comparativelysmall in dimensional shrinkage, to have less distortion, to be excellentin fusion between particles, and to be therefore good in appearance. Theresults are shown in Table 1.

Example 7

First-stage foaming, second-stage foaming, and the evaluation of in-moldexpansion molding were carried out in the same way as in Example 6,except that the additive, polyethylene glycol (having an averagemolecular weight of 600), was used in 0.5 parts by weight. Thefirst-stage expanded particles obtained by the first-stage foamingshowed two melting points and had an expansion ratio of 10 times, anopen cell ratio of 1.2%, and an average cell diameter d of 225 μm. Thecells were slightly inferior in uniformity to those of Examples 1 to 3,but were substantially uniform. The moisture content was 3.0%. Next,second-stage expanded particles having an expansion ratio of 30 timeswere obtained in the same way as in Example 1. The second-stage expandedparticles showed two melting points in differential scanningcalorimetry, had an open cell ratio of 2.5%, and were satisfactory inuniformity of cells with an average cell diameter d of 345 μm. As aresult of the evaluation of in-mold expansion molding, the in-moldexpanded molded product thus obtained was found to have a surfaceexcellent in smoothness, albeit with slight wrinkles and gaps betweenparticles, to be comparatively small in dimensional shrinkage, to haveless distortion, to be excellent in fusion between particles, and to betherefore good in appearance. The results are shown in Table 1.

Example 8

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 1, except thatcross-linked polyalkylene oxide was used in 1.0 part by weight insteadof polyethylene glycol. The cells in the first-stage expanded particleswere substantially uniform. The first-stage expanded particles wereprocessed into second-stage expanded particles, which were then used togive an in-mold expanded molded product. The in-mold expanded moldedproduct was slightly inferior in surface properties, slightly higher inrate of dimensional shrinkage, slightly lower in fusion betweenparticles in comparison with those obtained with use of polyethyleneglycol. The results are shown in Table 1.

Example 9

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 1, except that sodiumpolyacrylate was used in 0.5 parts by weight instead of polyethyleneglycol. The cells in the first-stage expanded particles were slightlynonuniform. The first-stage expanded particles were processed intosecond-stage expanded particles, which were then used to give an in-moldexpanded molded product. The in-mold expanded molded product wasslightly inferior in surface properties, slightly higher in rate ofdimensional shrinkage, and slightly lower in fusion between particles incomparison with those obtained with use of polyethylene glycol. Theresults are shown in Table 1.

Example 10

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 1, except thatcarboxymethylcellulose sodium was used as an additive in 0.3 parts byweight instead of polyethylene glycol and talc was used in 0.1 part byweight. The cells in the first-stage expanded particles were slightlynonuniform. The first-stage expanded particles were processed intosecond-stage expanded particles, which were then used to give an in-moldexpanded molded product. The in-mold expanded molded product wasslightly inferior in surface properties and slightly higher in rate ofdimensional shrinkage in comparison with those obtained with use ofpolyethylene glycol, but was satisfactory in fusion between particles.The results are shown in Table 1.

Example 11

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 1, except thatpolypropylene glycol (having an average molecular weight of 2,000) wasused in 0.2 parts by weight instead of polyethylene glycol and talc wasused in 0.1 part by weight. The cells in the first-stage expandedparticles were slightly nonuniform, and a few of the particlesagglomerated to each other during the second-stage foaming. Further, thesecond-stage expanded particles were used to give an in-mold expandedmolded product. The in-mold expanded molded product was slightlyinferior in surface properties, slightly higher in rate of dimensionalshrinkage, and slightly lower in fusion between particles in comparisonwith those obtained with use of polyethylene glycol. The results areshown in Table 1.

Example 12

Polyethylene glycol (having an average molecular weight of 300;manufactured by Lion Corporation) was pre-blended in 0.5 parts by weightto 100 parts by weight of a linear low-density polyethylene resin(MI=2.0 g/10 minutes, melting point of 122° C.). In addition, talc(manufactured as Talcan Powder PK-S by Hayashi-Kasei Co., Ltd.) wasblended as a foam nucleating agent in 0.1 part by weight. The mixturewas supplied to a single screw extruder 50 in diameter, melted andkneaded, extruded through a cylindrical die 1.8 mm in diameter, cooledwith water, and then cut with a cutter to give cylinder-shaped linearlow-density polyethylene resin particles (1.2 mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thecylinder-shaped linear low-density polyethylene resin particles thusobtained were fed together with 200 parts by weight of pure water, 1.0part by weight of tertiary calcium phosphate, and 0.05 parts by weightof soda dodecylbenzenesulfonate. After deairing, and while stirring, 12parts by weight of carbon dioxide were put into the closed vessel, whichwas then heated to 123° C. The internal pressure of thepressure-resistant closed vessel at this point in time was 4.5 MPa (gagepressure). Shortly after that, the water dispersion (the resin particlesand the aqueous dispersion medium) was released into a zone underatmospheric pressure through an orifice 3.6 mm in diameter by opening avalve installed in the lower part of the closed vessel, thus givingexpanded particles (first-stage expanded particles). During the release,the internal pressure of the vessel was retained by carbon dioxide so asnot to decrease.

The first-stage expanded particles thus obtained showed two meltingpoints of 117° C. and 128° C. in differential scanning calorimetry. As aresult of the measurement of the expansion ratio, the open cell ratio,and the average cell diameter, the first-stage expanded particles werefound to have an expansion ratio of 5 times and an open cell ratio of0.6%, to be excellent in uniformity of cells, and to have an averagecell diameter d of 160 p.m. The moisture content was measured by waterfoaming with the internal temperature of the closed vessel set at 123°C. as above, and was found to be 2.4%.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by drying them for six hour at 60° C., setting theinternal pressure at approximately 0.4 MPa (absolute pressure) throughimpregnating them with pressurized air in the closed vessel, andbringing them into contact with vapor of approximately 0.03 MPa (gagepressure), thus giving second-stage expanded particles having anexpansion ratio of 20 times. The second-stage expanded particles showedtwo melting points in differential scanning calorimetry, had an opencell ratio of 1.3%, and were excellent in uniformity of cells with anaverage cell diameter d of 270 μm. As a result of the microscopicobservation of the surfaces of the expanded particles subjected to thesecond-stage foaming, the expanded particles were found to be uniform indiameter of cells in the surfaces, to have smooth surfaces, and to besmall in number of thin parts of the surface membranes thereof. Next,the expanded particles subjected to the second-stage foaming weresubjected to in-mold expansion molding. The in-mold expanded moldedproduct thus obtained had a surface excellent in smoothness, albeit withslight wrinkles and gaps between particles, was comparatively small indimensional shrinkage, had less distortion, was excellent in fusionbetween particles, and therefore was good in appearance. The results areshown in Table 1.

Comparative Example 1

Foaming was carried out in the same way as in Example 1 under theconditions shown in the table, without use of polyethylene glycol. Theexpansion ratio thus attained was as low as 6 times, and the averagecell diameter was also as small as 150 μM. Further, in comparison withthe case of use of polyethylene glycol, a higher vapor pressure wasrequired in the second-stage foaming for an expansion ratio of 30 times;therefore, many of the expanded particles agglomerated to each other.Such second-stage expanded particles were subjected to in-mold expansionmolding to give an in-mold expanded molded product. The in-mold expandedmolded product was poor in surface properties, high in rate ofdimensional shrinkage, and inferior in appearance. The results are shownin Table 1.

Comparative Example 2

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 1, except that zeolite Awas used in 1.0 part by weight instead of polyethylene glycol and talcwas not used. The cells in the first-stage expanded particles wereinferior in uniformity because some cells were coarse and others weresmall. A few of the particles agglomerated to each other during thesecond-stage foaming, and a high vapor pressure was required for anexpansion ratio of 30 times; therefore, a few of the expanded particlesagglomerated to each other. Such second-stage expanded particles wereused to give an in-mold expanded molded product. The in-mold expandedmolded product was poor in surface properties and large in dimensionalshrinkage. The results are shown in Table 1.

Example 13

Glycerin (purified glycerin D; manufactured by Lion Corporation) waspre-blended in 0.1 part by weight to 100 parts by weight ofpolypropylene resin A (propylene-ethylene random copolymer: ethylenecontent of 3.0%, MI=6 g/10 minutes, melting point of 143° C.). Inaddition, talc (manufactured as Talcan Powder PK-S by Hayashi-Kasei Co.,Ltd.) was blended as a foam nucleating agent in 0.1 part by weight. Themixture was supplied to an extruder, melted and kneaded, extrudedthrough a cylindrical die 1.8 mm in diameter, cooled with water, andthen cut with a cutter to give cylinder-shaped polyolefin resinparticles (1.2 mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thepolyolefin resin particles thus obtained were fed together with 200parts by weight of pure water, 1.0 part by weight of tertiary calciumphosphate, and 0.05 parts by weight of soda dodecylbenzenesulfonate.After deairing, and while stirring, 6 parts by weight of carbon dioxidewere put into the closed vessel, which was then heated to 148° C. Theinternal pressure of the pressure-resistant closed vessel at this pointin time was 3 MPa (gage pressure). Shortly after that, the waterdispersion (the resin particles and the aqueous dispersion medium) wasreleased into a zone under atmospheric pressure through an orifice 4 mmin diameter by opening a valve installed in the lower part of the closedvessel, thus giving expanded particles (first-stage expanded particles).During the release, the internal pressure of the vessel was retained bycarbon dioxide so as not to decrease.

The first-stage expanded particles thus obtained showed two meltingpoints of 138° C. and 157° C. in differential scanning calorimetry. As aresult of the measurement of the expansion ratio, the open cell ratio,and the average cell diameter, the first-stage expanded particles werefound to have an expansion ratio of 15 times and an open cell ratio of0.9%, to be excellent in uniformity of cells, and to have an averagecell diameter d of 239 μm. The moisture content was measured by waterfoaming with the internal temperature of the closed vessel set at 148°C. as above, and was found to be 1.8%.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by drying them for six hour at 60° C., setting theinternal pressure at approximately 0.4 MPa (absolute pressure) throughimpregnating them with pressurized air in the closed vessel, andbringing them into contact with vapor of approximately 0.08 MPa (gagepressure), thus giving second-stage expanded particles having anexpansion ratio of 30 times. The second-stage expanded particles showedtwo melting points in differential scanning calorimetry, had an opencell ratio of 1.4%, and were excellent in uniformity of cells with anaverage cell diameter d of 300 μm. Next, the expanded particlessubjected to the second-stage foaming were subjected to in-moldexpansion molding after setting the internal air pressure atapproximately 0.19 MPa (absolute pressure) through again pressuring themwith air in the closed vessel. The in-mold expanded molded product thusobtained had a surface excellent in smoothness, albeit with slightwrinkles and gaps between particles, was comparatively low in rate ofdimensional shrinkage, had less distortion, was excellent in fusionbetween particles, and therefore was good in appearance. The results areshown in Table 2.

TABLE 2 Examples 13 14 15 16 17 18 19 20 21 22 Polypropylene resin AParts by 100 100 100 100 100 100 100 100 100 weight Linear low-densitypolyethylene resin Parts by 100 weight Glycerin Parts by 0.1 0.15 0.20.05 0.2 0.2 0.2 0.2 weight Diglycerin Parts by 0.2 weight Polyethyleneglycol Parts by 0.1 (avg. molecular wt.: 300) weight Melamine Parts by0.05 weight Polyethylene glycol dimethyl ether Parts by 0.5 weightCross-linked polyalkylene oxide Parts by weight Sodium polyacrylateParts by weight Carboxymethylcellulose sodium Parts by weight ZeoliteParts by weight Polypropylene glycol Parts by weight Talc Parts by 0.10.05 0.02 0.05 0.05 0.02 0.05 0.02 0.1 0.1 weight Percentage of moistruecontent % 1.8 2.4 2.9 0.9 3.3 3.3 2.0 2.9 0.4 2.4 Amount of CO₂ Parts by6 6 6 3 6 6 6 0 6 12 weight First-stage Expansion ratio Times 15 18 17 717 16 15 13 7 6 expanded Average cell μm 239 250 271 210 304 260 250 190250 170 particles diameter d Uniformity of cells Very Very Good GoodVery Good Good Good Good Good good good good Second-stage expandabilityGood Good Good Good Good Good Good Good Average Good Molded Surfaceproperties Good Good Very Good Very Good Good Good Average Good Productgood good Rate of dimensional Good Very Very Good Very Good Good GoodAverage Good shrinkage good good good Rate of fusion % Very Very VeryVery Very Very Very Very Good Very good good good good good good goodgood good

Example 14

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that the additive,glycerin, and talc were used in 0.15 parts by weight and 0.05 parts byweight, respectively. The first-stage expanded particles showed twomelting points, had an expansion ratio of 18 times and an open cellratio of 1.2%, were excellent in uniformity of cells, and had an averagecell diameter d of 250 μm. The moisture content was 2.4%. Next,second-stage expanded particles having an expansion ratio of 30 timeswere obtained in the same way as in Example 13. The second-stageexpanded particles showed two melting points in differential scanningcalorimetry, had an open cell ratio of 1.3%, and were excellent inuniformity of cells with an average cell diameter d of 290 μm. As aresult of the in-mold expansion molding, the in-mold expanded moldedproduct thus obtained had a surface excellent in smoothness, albeit withslight wrinkles and gaps between particles, was small in dimensionalshrinkage, to have less distortion, was excellent in fusion betweenparticles, and therefore was good in appearance. The results are shownin Table 2.

Example 15

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that the additive,glycerin, and talc were used in 0.2 parts by weight and 0.02 parts byweight, respectively. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points, had an expansion ratio of17 times, an open cell ratio of 0.7%, and an average cell diameter d of271 μm. The cells were slightly inferior in uniformity to those ofExample 13, but were substantially uniform. The moisture content was2.9%. Next, second-stage expanded particles having an expansion ratio of30 times were obtained in the same way as in Example 13. Thesecond-stage expanded particles showed two melting points indifferential scanning calorimetry, had an open cell ratio of 1.0%, andwere satisfactory in uniformity of cells with an average cell diameter dof 330 μm. As a result of the in-mold expansion molding, the in-moldexpanded molded product thus obtained had a surface excellent insmoothness and free of wrinkles, was small in dimensional shrinkage, hadless distortion, was excellent in fusion between particles, andtherefore was good in appearance. The results are shown in Table 2.

Example 16

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that glycerin,talc, and carbon dioxide were used in 0.05 parts by weight, 0.05 partsby weight, and 3 parts by weight, respectively. The first-stage expandedparticles obtained by the first-stage foaming showed two melting points,had an expansion ratio of 7 times, an open cell ratio of 0.7%, and anaverage cell diameter d of 210 μm. The cells were slightly inferior inuniformity to those of Example 13, but were substantially uniform. Themoisture content was 0.9%. Next, second-stage expanded particles havingan expansion ratio of 30 times were obtained in the same way as inExample 13. The second-stage expanded particles showed two meltingpoints in differential scanning calorimetry, had an open cell ratio of1.0%, and were satisfactory in uniformity of cells with an average celldiameter d of 280 μm. As a result of the in-mold expansion molding, thein-mold expanded molded product thus obtained had a surface excellent insmoothness, albeit with slight wrinkles and gaps between particles, wascomparatively small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. The results are shown in Table 2.

Example 17

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that polyethyleneglycol (having an average molecular weight of 300; manufactured by LionCorporation) and talc were used in 0.1 part by weight and 0.05 parts byweight, respectively, in addition to the additive, glycerin, used in 0.2parts by weight. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points, had an expansion ratio of17 times, an open cell ratio of 1.1%, and an average cell diameter d of304 μm. The cells were particularly superior in uniformity to those ofExamples 13 to 15. The moisture content was 3.3%. Next, second-stageexpanded particles having an expansion ratio of 30 times were obtainedin the same way as in Example 13. The second-stage expanded particlesshowed two melting points in differential scanning calorimetry, had anopen cell ratio of 1.5%, and were excellent in uniformity of cells withan average cell diameter d of 365 μm. As a result of the in-moldexpansion molding, the in-mold expanded molded product thus obtained hada surface excellent in smoothness and free of wrinkles, was small indimensional shrinkage, had less distortion, was excellent in fusionbetween particles, and therefore was good in appearance. The results areshown in Table 2.

Example 18

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that melamine(pulverized melamine; manufactured by Nissan Chemical Industries, Ltd.)and talc were used in 0.05 parts by weight and 0.02 parts by weight,respectively, in addition to the additive, glycerin, used in 0.2 partsby weight. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points, had an expansion ratio of16 times, an open cell ratio of 1.0%, and an average cell diameter d of260 μm. The cells were slightly inferior in uniformity to those ofExample 13, but were substantially uniform. The moisture content was3.3%. Next, second-stage expanded particles having an expansion ratio of30 times were obtained in the same way as in Example 13. Thesecond-stage expanded particles, showed two melting points indifferential scanning calorimetry, had an open cell ratio of 1.5%, andwere excellent in uniformity of cells with an average cell diameter d of320 μm. As a result of the in-mold expansion molding, the in-moldexpanded molded product thus obtained had a surface excellent insmoothness, albeit with slight wrinkles and gaps between particles, wascomparatively small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. The results are shown in Table 2.

Example 19

Foaming, second-stage foaming, and in-mold expansion molding werecarried out in the same way as in Example 13, except that diglycerin wasused as an additive in 0.2 parts by weight and talc was used in 0.05parts by weight. The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points, had an expansion ratio of15 times, an open cell ratio of 1.4%, and an average cell diameter d of250 μm. The cells were slightly inferior in uniformity to those ofExample 13, but were substantially uniform. The moisture content was2.0%. Next, second-stage expanded particles having an expansion ratio of30 times were obtained in the same way as in Example 13. Thesecond-stage expanded particles showed two melting points indifferential scanning calorimetry, had an open cell ratio of 1.5%, andwere excellent in uniformity of cells with an average cell diameter d of305 μm. As a result of the in-mold expansion molding, the in-moldexpanded molded product thus obtained had a surface excellent insmoothness, albeit with slight wrinkles and gaps between particles, wascomparatively small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. The results are shown in Table 2.

Example 20

The additive, glycerin, was used in 0.2 parts by weight, and talc wasused in 0.02 part by weight. The foaming agent, carbon dioxide, was notused, but nitrogen gas was introduced instead. The temperature wasraised to 151° C. As for the rest, first-stage foaming, second-stagefoaming, and the evaluation of in-mold expansion molding were carriedout in the same way as in Example 13. The internal pressure of theclosed vessel during the first-stage foaming was 3.0 MPa (gagepressure). The first-stage expanded particles obtained by thefirst-stage foaming showed two melting points and had an expansion ratioof 13 times, an open cell ratio of 1.5%, and an average cell diameter dof 190 μm. The cells were slightly inferior in uniformity to those ofExample 13, but were substantially uniform. The moisture content was2.9%. Next, second-stage expanded particles having an expansion ratio of30 times were obtained in the same way as in Example 13. Thesecond-stage expanded particles showed two melting points indifferential scanning calorimetry, had an open cell ratio of 2.3%, andwere excellent in uniformity of cells with an average cell diameter d of250 μm. As a result of the in-mold expansion molding, the in-moldexpanded molded product thus obtained had a surface excellent insmoothness, albeit with slight wrinkles and gaps between particles, wascomparatively small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. The results are shown in Table 2.

Example 21

First-stage foaming, second-stage foaming, and in-mold expansion moldingwere carried out in the same way as in Example 13, except thatpolyethylene glycol dimethyl ether was used in 0.5 parts by weightinstead of glycerin and talc was used in 0.1 part by weight. Thefirst-stage expanded particles had a low expansion ratio of 7 times. Thecells were slightly inferior in uniformity to those of Example 13, butwere substantially uniform. A few of the particles agglomerated to eachother during the second-stage foaming. Such second-stage expandedparticles were subjected to the in-mold expansion molding to give anin-mold expanded molded product. The in-mold expanded molded product wasslightly inferior in surface properties and slightly higher in rate ofdimensional shrinkage in comparison with that of Example 13, in whichglycerin was used. The in-mold expanded molded product was satisfactoryin fusion between particles, but there were slightly more unfusedparticles than in Example 13. The results are shown in Table 2.

References below describe a method for determining whether or not anadditive is a water-absorbing substance of the present invention havingno function of forming foaming nuclei.

Example 22

Glycerin (purified glycerin D; manufactured by Lion Corporation) waspre-blended in 0.2 parts by weight to 100 parts by weight of a linearlow-density polyethylene resin (MI=2.0 g/10 minutes, melting point of122° C.). In addition, talc (manufactured as Talcan Powder PK-S byHayashi-Kasei Co., Ltd.) was blended as a foam nucleating agent in 0.1part by weight. The mixture was supplied to a single screw extruder 50in diameter, melted and kneaded, extruded through a cylindrical die 1.8mm in diameter, cooled with water, and then cut with a cutter to givecylinder-shaped linear low-density polyethylene resin particles (1.2mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thecylinder-shaped linear low-density polyethylene resin particles thusobtained were fed together with 200 parts by weight of pure water, 1.0part by weight of tertiary calcium phosphate, and 0.05 parts by weightof soda dodecylbenzenesulfonate. After deairing, and while stirring, 12parts by weight of carbon dioxide were put into the closed vessel, whichwas then heated to 123° C. The internal pressure of thepressure-resistant closed vessel at this point in time was 4.5 MPa (gagepressure). Shortly after that, the water dispersion (the resin particlesand the aqueous dispersion medium) was released into a zone underatmospheric pressure through an orifice 3.6 mm in diameter by opening avalve installed in the lower part of the closed vessel, thus givingexpanded particles (first-stage expanded particles). During the release,the internal pressure of the vessel was retained by carbon dioxide so asnot to decrease.

The first-stage expanded particles thus obtained showed two meltingpoints of 117° C. and 128° C. in differential scanning calorimetry. As aresult of the measurement of the expansion ratio, the open cell ratio,and the average cell diameter, the first-stage expanded particles werefound to have an expansion ratio of 6 times and an open cell ratio of0.6%, to be excellent in uniformity of cells, and to have an averagecell diameter d of 160 μm. The moisture content was measured by waterfoaming with the internal temperature of the closed vessel set at 123°C. as above, and was found to be 2.4%.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by drying them for six hour at 60° C., setting theinternal pressure at approximately 0.4 MPa (absolute pressure) throughimpregnating them with pressurized air in the closed vessel, andbringing them into contact with vapor of approximately 0.03 MPa (gagepressure), thus giving second-stage expanded particles having anexpansion ratio of 20 times. The second-stage expanded particles showedtwo melting points in differential scanning calorimetry, had an opencell ratio of 1.3%, and were excellent in uniformity of cells with anaverage cell diameter d of 270 μm. As a result of the microscopicobservation of the surfaces of the expanded particles subjected tosecond-stage foaming, the expanded particles were found to be uniform indiameter of cells in the surfaces, to have smooth surfaces, and to besmall in number of thin parts of the surface membranes thereof. Next,the expanded particles subjected to the second-stage foaming weresubjected to in-mold expansion molding. The in-mold expanded moldedproduct thus obtained had a surface excellent in smoothness, albeit withslight wrinkles and gaps between particles, was comparatively small indimensional shrinkage, had less distortion, was excellent in fusionbetween particles, and therefore was good in appearance. The results areshown in Table 2.

Reference 1

Polypropylene resin B (propylene/ethylene/butene-1 random copolymer:ethylene content of 2.6% by weight, butene-1 content of 1.4% by weight,melt index of 7 g/10 minutes, melting point of 145° C.) was supplied toa single screw extruder 50 in diameter, melted and kneaded at 200° C.,extruded through a cylindrical die 1.8 mm in diameter, cooled withwater, and then cut with a cutter to give cylinder-shaped polypropyleneresin particles (1.2 mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thecylinder-shaped polypropylene resin particles thus obtained were fedtogether with 300 parts by weight of pure water, 1.0 part by weight oftertiary calcium phosphate, and 0.05 parts by weight of sodadodecylbenzenesulfonate. After deairing, and while stirring, 14 parts byweight of carbon dioxide were put into the closed vessel, which was thenheated to 149° C. The internal pressure of the closed vessel at thispoint in time was 3 MPa (gage pressure). Shortly after that, the waterdispersion containing polypropylene resin particles and the aqueousdispersion medium was released into a zone under atmospheric pressurethrough an orifice 4 mm in diameter by opening a valve installed in thelower part of the closed vessel, thus giving expanded polypropyleneresin particles. During the release, the internal pressure of the vesselwas retained by carbon dioxide so as not to decrease.

The additive-free expanded polypropylene resin particles thus obtainedhad an average cell diameter L(av) of 260 μm.

Reference 2

Polyethylene glycol (having an average molecular weight of 300;manufactured by Lion Corporation) was pre-blended as an additive D in0.5 parts by weight to 100 parts by weight of polypropylene resin B. Themixture was supplied to a single screw extruder 50 in diameter, meltedand kneaded at 200° C., extruded through a cylindrical die 1.8 mm indiameter, cooled with water, and then cut with a cutter to givecylinder-shaped polyethylene glycol-containing polypropylene resinparticles (1.2 mg/particle).

After that, expanded polyethylene glycol-containing polypropylene resinparticles were produced in just the same way as in Reference 1. Theaverage cell diameter L(av) of the expanded particles thus obtained isshown in Table 3. A comparison with Reference 1 shows that polyethyleneglycol is a water-absorbing substance of the present invention having nofunction of forming foaming nuclei.

TABLE 3 Avg. cell Water-absorbing substance Additives Melting pointdiameter having no function of References Codes Types (° C.) L(av)forming foaming nuclei 1 N/A N/A N/A 260 N/A 2 D Polyethylene glycol(avg. molecular wt.: 300) −13 230 Yes 3 E Block copolymer containing apolyolefin block 135 200 Yes and a polyethylene oxide block (Pelestat303) 4 F Sodium Polyacrylate Decomposed 300 Yes before melting 5 GCarboxymethylcellulose sodium No melting point 210 Yes 6 H Polyvinylalcohol 150-230 200 Yes 7 I Bentonite 1,000 or higher 260 Yes 8 JSynthetic hectolite (300 or higher) 290 Yes 9 K Synthetic zeolite (300or higher) 190 Yes 10 L Zinc borate 980 140 No * N/A means “notapplicable.”

Reference 3

Expanded polypropylene resin particles were produced in just the sameway as in Reference 2, except that a block copolymer having a polyolefinblock and a polyethylene oxide block (Pelestat 303; manufactured bySanyo Chemical Industries, Ltd.) was used as an additive E instead ofthe additive D, polyethylene glycol. The average cell diameter L(av) ofthe expanded particles thus obtained is shown in Table 3. A comparisonwith Reference 1 shows that the block copolymer having a polyolefinblock and a polyethylene oxide block is a water-absorbing substance ofthe present invention having no function of forming foaming nuclei.

References 4 to 9

Expanded polypropylene resin particles were produced in just the sameway as in Reference 2, except that each of the following additives F toK was used instead of the additive D, polyethylene glycol. Thepercentage of volatile content and average cell diameter L(av) of theexpanded particles thus obtained are shown in Table 3. It is shown thateach of the additives is a water-absorbing substance of the presentinvention having no function of forming foaming nuclei.

Additives

F: Sodium polyacrylate (AQUA KEEP 10SH-NF; manufactured by SumitomoSeika Chemicals Co., Ltd.)

G: Carboxymethylcellulose sodium (MAC20; manufacture by Nippon PaperChemicals Co., Ltd.)

H: Polyvinyl alcohol (PVA205S; manufactured by Kuraray Co., Ltd.)

I: Bentonite (Bengel Bright 25; manufactured by Hojun)

J: Synthetic hectolite (Laponite RD; manufactured by Toshin ChemicalsCo., Ltd.)

K: Synthetic zeolite (NX-100P; manufactured by Nippon ChemicalIndustrial Co., Ltd.)

Reference 10

Expanded polypropylene resin particles were produced in just the sameway as in Reference 3, except that zinc borate (zinc borate 2335;manufactured by Tomita Pharmaceuticals Co., Ltd.) was used as anadditive L instead of the additive D, polyethylene glycol. The averagecell diameter L(av) of the expanded particles is shown in Table 3. It isshown that this additive is not a water-absorbing substance of thepresent invention having no function of forming foaming nuclei.

Example 23

The additive D (polyethylene glycol having an average molecular weightof 300; manufactured by Lion Corporation) was pre-blended in 0.5 partsby weight to 100 parts by weight of polypropylene resin(propylene/ethylene/butene-1 random copolymer: ethylene content of 2.6%by weight, butene-1 content of 1.4% by weight, melt index of 7 g/10minutes, melting point of 145° C.). In addition, talc (manufactured asTalcan Powder PK-S by Hayashi-Kasei Co., Ltd.) was blended as a foamnucleating agent in 0.03 part by weight. The mixture was supplied to asingle screw extruder 50 in diameter, melted and kneaded with atemperature of 200° C. at the tip of the die, extruded through acylindrical die 1.8 mm in diameter, cooled with water, and then cut witha cutter to give cylinder-shaped polypropylene resin particles (1.2mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thecylinder-shaped polypropylene resin particles thus obtained were fedtogether with 300 parts by weight of pure water, 2.0 parts by weight oftertiary calcium phosphate, and 0.05 parts by weight of sodadodecylbenzenesulfonate. After deairing, and while stirring, 14 parts byweight of carbon dioxide were put into the closed vessel, which was thenheated to 149° C. The internal pressure of the closed vessel at thispoint in time was 2.9 MPa (gage pressure). Furthermore, the internaltemperature of the closed vessel was set at 3.3 MPa (gage pressure) byadding carbon dioxide gases, and retained for 10 minutes. After that,water dispersion (the resin particles and the aqueous dispersion medium)was released into a zone under atmospheric pressure through an orifice 4mm in diameter by opening a valve installed in the lower part of theclosed vessel, thus giving expanded polypropylene resin particles(first-stage expanded particles). During the release, the internalpressure of the vessel was retained by carbon dioxide so as not todecrease during the release.

The first-stage expanded particles thus obtained showed two endothermicpeaks at approximately 142° C. and approximately 159° C. in differentialscanning calorimetry. As a result of the measurement of the expansionratio, the open cell ratio, and the average cell diameter L(av), thefirst-stage expanded particles were found, as shown in Table 4, to havean expansion ratio of 15 times, an open cell ratio of 0.6%, a percentageof volatile content of 3.0%, and an average cell diameter L(av) of 270μm. The first-stage expanded particles had a small cell diametervariation S of 0.07 and therefore were excellent in uniformity in celldiameter.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by cleansing them with acid, drying them for sixhour at 60° C., setting the internal pressure at approximately 0.4 MPa(absolute pressure) through impregnating them with pressurized air inthe closed vessel, and bringing them into contact with vapor ofapproximately 0.07 MPa (gage pressure), thus giving second-stageexpanded particles having an expansion ratio of 30 times. As a result ofthe microscopic observation of the surfaces of the expanded particlessubjected to the second-stage foaming, the expanded particles were foundto be uniform in diameter of cells in the surfaces, to have smoothsurfaces, and to be small in number of thin parts of the surfacemembranes thereof. Next, the expanded particles subjected to thesecond-stage foaming were subjected to in-mold expansion molding aftersetting the internal air pressure at approximately 0.2 MPa (absolutepressure) through again pressuring them with air in the closed vessel.The in-mold expanded molded product thus obtained had a surfaceexcellent in smoothness and free of wrinkles, was small in dimensionalshrinkage, had less distortion, was excellent in fusion betweenparticles, and therefore was good in appearance. The results are shownin Table 4.

TABLE 4 Examples 23 24 25 26 27 28 29 30 31 32 33 Polyropylene resin B BB B B B B B C C C Type of additives D D F F G H J K D E I Amount ofadditives Parts by weight 0.5 0.5 0.2 0.5 0.5 0.5 1.0 1.0 0.5 1.0 1.0Talc Parts by weight 0.03 0.30 0.05 0.05 0.03 0.03 0.03 0.03 0.05 0.050.01 Initial amount of CO₂ Parts by weight 14 14 14 14 14 14 14 14 6 6 6Expansion ratio Times 15 17 11 18 11 11 13 12 19 12 14 Open cell ratio %0.6 0.7 1.0 1.5 1.7 0.6 1.8 1.7 0.6 0.9 1.1 Percentage of volatile % 3.03.0 2.2 3.6 5.8 2.9 2.9 4.3 3.1 1.8 6.0 content Avg. cell diameter μm270 230 310 410 280 240 280 280 350 350 290 L(av) Cell diametervariation S 0.07 0.05 0.2 0.4 0.4 0.2 0.15 0.1 0.05 0.05 0.4 Shrinkageand wrinkles in expanded Good Good Good Good Good Good Good Good GoodGood Good particles Internal pressure MPa approx. approx. approx.approx. approx. approx. approx. approx. approx. approx. approx. duringsecond- (absolute pressure) 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4 0.4stage foaming Vapor pressure MPa (gage pressure) approx. approx. approx.approx. approx. approx. approx. approx. approx. approx. approx. duringsecond- 0.07 0.07 0.08 0.07 0.08 0.08 0.08 0.08 0.06 0.08 0.07 stagefoamng Second-stage expandability Good Good Good Good Good Good GoodGood Good Good Good Surface properties of molded product Very Very VeryGood Good Very Very Very Very Very Good good good good good good goodgood good Rate of fusion of molded product Very Very Very Good Good VeryVery Good Very Very Very good good good good good good good good

Example 24

First-stage expanded particles were obtained in the same way as inExample 23, except that the foam nucleating agent, talc, was used in 0.3parts by weight and the second-stage foaming conditions were as shown inTable 4. Next, second-stage expanded particles having an expansion ratioof 30 times were obtained and subjected to in-mold expansion molding.The first-stage expanded particles thus obtained showed two endothermicpeaks at approximately 142° C. and approximately 159° C. in differentialscanning calorimetry, and the expansion ratio, the open cell ratio, theaverage cell diameter L(av), and the cell diameter variation S were asshown in Table 4. As a result of the microscopic observation of thesurfaces of the expanded particles subjected to the second-stagefoaming, the expanded particles were found to be uniform in diameter ofcells in the surfaces, to have smooth surfaces, and to be small innumber of thin parts of the surface membranes thereof. The second-stageexpanded particles were subjected to the in-mold expansion molding togive an in-mold expanded molded product. The in-mold expanded moldedproduct thus obtained had a surface excellent in smoothness and free ofwrinkles, was small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. The results are shown in Table 4.

Examples 25 to 30

First-stage expanded particles were obtained in the same way as inExample 23, except that each of the additives F, G, H, J, and K was usedinstead of the additive D, polyethylene glycol, in an amount shown inTable 4, that the foam nucleating agent, talc, was used in an amountshown in Table 4, and that the second-stage foaming conditions were asshown in Table 4. Next, second-stage expanded particles having anexpansion ratio of 30 times were obtained and subjected to in-moldexpansion molding. The first-stage expanded particles thus obtainedshowed two endothermic peaks at approximately 142° C. and approximately159° C. in differential scanning calorimetry, and the expansion ratio,the open cell ratio, the average cell diameter L(av), and the celldiameter variation S were as shown in Table 4. As a result of themicroscopic observation of the surfaces of the expanded particlessubjected to the second-stage foaming, the expanded particles were foundto be uniform in diameter of cells in the surfaces, to have smoothsurfaces, and to be small in number of thin parts of the surfacemembranes thereof. The second-stage expanded particles were subjected tothe in-mold expansion molding to give an in-mold expanded moldedproduct. The in-mold expanded molded product thus obtained had a surfaceexcellent in smoothness and free of wrinkles, was small in dimensionalshrinkage, had less distortion, was excellent in fusion betweenparticles, and therefore was good in appearance. However, in Examples 26and 27, the in-mold expanded molded products showed slight wrinkles andgaps between particles on their surfaces. The in-mold expanded moldedproducts were satisfactory in fusion between particles, but there wereslightly more unfused particles. Also, in Example 30, the in-moldexpanded molded product was satisfactory in fusion between particles,but there were slightly more unfused particles.

Example 31

The additive D (polyethylene glycol having an average molecular weightof 300; manufactured by Lion Corporation) was pre-blended in 0.5 partsby weight to 100 parts by weight of polypropylene resin C(propylene/ethylene random copolymer: ethylene content of 3.2% byweight, melt index of 6 g/10 minutes, melting point of 142° C.). Inaddition, talc (manufactured as Talcan Powder PK-S by Hayashi-Kasei Co.,Ltd.) was blended as a foam nucleating agent in 0.05 part by weight. Themixture was supplied to a single screw extruder 50 in diameter, meltedand kneaded with a temperature of 200° C. at the tip of the die,extruded through a cylindrical die 1.8 mm in diameter, cooled withwater, and then cut with a cutter to give cylinder-shaped polyolefinresin particles (1.2 mg/particle).

Into a pressure-resistant closed vessel, 100 parts by weight of thepolypropylene resin particles thus obtained were fed together with 200parts by weight of pure water, 2.0 part by weight of tertiary calciumphosphate, and 0.05 parts by weight of soda dodecylbenzenesulfonate.After deairing, and while stirring, 6 parts by weight of carbon dioxidewere put into the closed vessel, which was then heated to 148° C. Theinternal pressure of the closed vessel at this point in time was 2.8 MPa(gage pressure). Furthermore, the internal temperature of the closedvessel was set at 3.0 MPa (gage pressure) by adding carbon dioxidegasses, and retained for ten minutes. After that, the water dispersioncontaining polypropylene resin particles and the aqueous dispersionmedium was released into a zone under atmospheric pressure through anorifice 4 mm in diameter by opening a valve installed in the lower partof the closed vessel, thus giving expanded polypropylene resin particles(first-stage expanded particles). During the release, the internalpressure of the vessel was retained by carbon dioxide so as not todecrease.

The first-stage expanded particles thus obtained showed two endothermicpeaks at approximately 138° C. and approximately 157° C. in differentialscanning calorimetry. As a result of the measurement of the expansionratio, the open cell ratio, and the average cell diameter L(av), thefirst-stage expanded particles were found, as shown in Table 4, to havean expansion ratio of 19 times, an open cell ratio of 0.6%, a percentageof volatile content of 3.1%, and an average cell diameter L(av) of 350μm. The first-stage expanded particles had a small cell diametervariation S of 0.05 and therefore were excellent in uniformity in celldiameter.

The first-stage expanded particles thus obtained were subjected tosecond-stage foaming by cleansing them with acid, drying them for sixhour at 60° C., setting the internal pressure at approximately 0.4 MPa(absolute pressure) through impregnating them with pressurized air inthe closed vessel, and bringing them into contact with vapor ofapproximately 0.06 MPa (gage pressure), thus giving second-stageexpanded particles having an expansion ratio of 30 times. As a result ofthe microscopic observation of the surfaces of the expanded particlessubjected to second-stage foaming, the expanded particles were found tobe uniform in diameter of cells in the surfaces, to have smoothsurfaces, and to be small in number of thin parts of the surfacemembranes thereof. Next, the expanded particles subjected to thesecond-stage foaming were subjected to in-mold expansion molding aftersetting the internal air pressure at approximately 0.2 MPa (absolutepressure) through again pressuring them with air in the closed vessel.The in-mold expanded molded product thus obtained had a surfaceexcellent in smoothness and free of wrinkles, was small in dimensionalshrinkage, had less distortion, was excellent in fusion betweenparticles, and therefore was good in appearance. The results are shownin Table 4.

Examples 32 and 33

First-stage expanded particles were obtained in the same way as inExample 31, except that each of the additives E and I was used insteadof the additive D, polyethylene glycol, in an amount shown in Table 4,that the foam nucleating agent, talc, was used in an amount shown inTable 4, and that the second-stage foaming conditions were as shown inTable 4. Next, second-stage expanded particles having an expansion ratioof 30 times were obtained and subjected to in-mold expansion molding.The first-stage expanded particles thus obtained showed two endothermicpeaks at approximately 138° C. and approximately 157° C. in differentialscanning calorimetry, and the expansion ratio, the open cell ratio, theaverage cell diameter L(av), and the cell diameter variation S were asshown in Table 4. As a result of the microscopic observation of thesurfaces of the expanded particles subjected to the second-stagefoaming, the expanded particles were found to be uniform in diameter ofcells in the surfaces, to have smooth surfaces, and to be small innumber of thin parts of the surface membranes thereof. The second-stageexpanded particles were subjected to the in-mold expansion molding togive an in-mold expanded molded product. The in-mold expanded moldedproduct thus obtained had a surface excellent in smoothness and free ofwrinkles, was small in dimensional shrinkage, had less distortion, wasexcellent in fusion between particles, and therefore was good inappearance. However, in Example 33, the in-mold expanded molded productsshowed slight wrinkles and gaps between particles on their surfaces. Thein-mold expanded molded products were satisfactory in fusion betweenparticles. The results are shown in Table 4.

Comparative Example 3

First-stage expanded particles were obtained in the same way as inExample 23, except that the additive D was not used and the second-stagefoaming conditions were as shown in Table 5. Next, second-stage expandedparticles having an expansion ratio of 30 times were obtained andsubjected to in-mold expansion molding. The first-stage expandedparticles thus obtained showed two endothermic peaks at approximately142° C. and approximately 159° C. in differential scanning calorimetry,and the expansion ratio, the open cell ratio, the average cell diameterL(av), and the cell diameter variation S were as shown in Table 5. Inparticular, the first-stage expanded particles had a cell diametervariation S of 0.5 and therefore were poor in uniformity in celldiameter and varied in size. A high vapor pressure was required in thesecond-stage foaming for an expansion ratio of 30 times; therefore, afew of the expanded particles agglomerated to each other. Suchsecond-stage expanded particles were subjected to in-mold expansionmolding to give an in-mold expanded molded product. The in-mold expandedmolded product was high in rate of dimensional shrinkage, had wrinklesand gaps between particles, and was inferior in appearance. The resultsare shown in Table 5.

TABLE 5 Comparative Examples 3 4 5 6 Polyropylene resin B B B C Type ofadditives N/A H L N/A Amount of additives Parts by weight N/A 1.0 1.0N/A Talc Parts by weight 0.03 N/A N/A 0.05 Initial amount of Parts byweight 14 14 14 6 CO₂ Expansion ratio Times 10 5 14 7 Open cell ratio %0.6 1.1 2.5 0.9 Percentage of % 1.2 3.8 6.3 0.9 volatile content Avg.cell diameter μm 280 320 120 210 L(av) Cell diameter variation S 0.5 0.60.05 0.35 Shrinkage and wrinkles in expanded Good Good Poor Goodparticles Internal pressure MPa approx. 0.4 approx. 0.6 approx. 0.4approx. 0.6 during second- (absolute pressure) stage foaming Vaporpressure MPa (gage pressure) approx. 0.09 approx. 0.11 approx. 0.07approx. 0.10 during second- stage foamng Second-stage expandabilityAverage Poor Good Poor Surface properties of molded product Poor PoorPoor Average Rate of fusion of molded product Good Average Poor Good

Comparative Examples 4 and 5

First-stage expanded particles were obtained in the same way as inExample 23, except that each of the additives H and L was used insteadof the additive D, polyethylene glycol, in an amount shown in Table 5,that the foam nucleating agent, talc, was used in an amount shown inTable 5, and that the second-stage foaming conditions were as shown inTable 5. Next, second-stage expanded particles having an expansion ratioof 30 times were obtained and subjected to in-mold expansion molding.

The first-stage expanded particles obtained in Comparative Example 4,where the additive H was used and no talc was added, showed twoendothermic peaks at approximately 142° C. and approximately 159° C. indifferential scanning calorimetry, and the expansion ratio, the opencell ratio, the average cell diameter L(av), and the cell diametervariation S were as shown in Table 5. In particular, the first-stageexpanded particles had a cell diameter variation S of 0.6 and thereforewere poor in uniformity in cell diameter and varied in size. A highvapor pressure was required in the second-stage foaming for an expansionratio of 30 times; therefore, many of the expanded particlesagglomerated to each other. Such second-stage expanded particles weresubjected to in-mold expansion molding to give an in-mold expandedmolded product. The in-mold expanded molded product was high in rate ofdimensional shrinkage, had wrinkles, and was inferior in appearance.

In Comparative Example 5, where the additive L was used and no talc wasadded, the first-stage expanded particles were satisfactory inuniformity in cell diameter, but had a small average cell diameterL(av). The expanded particles had many wrinkles, and were shrunk.Furthermore, as a result of in-mold expansion molding, the in-moldexpanded molded product had wrinkles and were inferior in fusibility.The results are shown in Table 5.

Comparative Example 6

First-stage expanded particles were obtained in the same way as inExample 31, except that the additive D was not used and the second-stagefoaming conditions were as shown in Table 5. Next, second-stage expandedparticles having an expansion ratio of 30 times were obtained andsubjected to in-mold expansion molding. The first-stage expandedparticles thus obtained showed two melting points of approximately 138°C. and approximately 157° C. in differential scanning calorimetry, andthe expansion ratio, the open cell ratio, the average cell diameterL(av), and the cell diameter variation S were as shown in Table 5. Ahigh vapor pressure was required in the second-stage foaming for anexpansion ratio of 30 times; therefore, many of the expanded particlesagglomerated to each other. Such second-stage expanded particles weresubjected to in-mold expansion molding to give an in-mold expandedmolded product. The in-mold expanded molded product was high in rate ofdimensional shrinkage, had surface sinks in addition to wrinkles andgaps between particles, and was inferior in appearance. The results areshown in Table 5.

Example 34

Foaming was carried out in the same way as in Example 23, except thatthe additive D, polyethylene glycol, was added in 0.1 part by weight,thus giving first-stage expanded particles having two endothermic peaksat approximately 142° C. and approximately 159° C. in differentialscanning calorimetry. The first-stage expanded particles had anexpansion ratio of 9 times and an average cell diameter L(av) of 250 μm.FIG. 2 shows a relationship between the expansion ratio and the averagecell diameter, including the results of Example 23. The first-stageexpanded particles obtained as mentioned above and the first-stageexpanded particles obtained in Example 23 were subjected to in-moldexpansion molding after setting the internal air pressure atapproximately 0.2 MPa (absolute pressure) through pressuring them withair in their respective pressure-resistant closed vessels. The rate offusion of each in-mold expanded molded product was evaluated. Theresults are shown in Table 6.

In this example, the average cell diameter varies only slightly inresponse to a change in the expansion ratio. This shows that theexpansion ratio can be controlled without influence of a change in theaverage cell diameter. Further, it is shown that an increase in theexpansion ratio does not lead to degradation in fusibility of thein-mold expanded molded product.

TABLE 6 Example 34 Comp. Example 7 Additives D L Amount of Parts by 0.10.5 0.01 0.1 additives weight Talc Parts by 0.03 0.03 — — weightExpansion ratio Times 9 15 8 15 Avg. cell μm 250 270 260 190 diameter L(av) Rate of fusion of Very good Very good Very good Average moldedproduct

Comparative Example 7

Foaming was carried out in the same way as in Comparative Example 5,except that the additive L, zinc borate, was added in 0.01 part byweight or in 0.1 part by weight, thus giving first-stage expandedparticles having two endothermic peaks at approximately 142° C. andapproximately 159° C. in differential scanning calorimetry. When theadditive L was added in 0.01 part by weight, the expansion ratio was 8times and the average cell diameter L(av) was 260 μm. When the additiveL was added in 0.1 part by weight, the expansion ratio was 15 times andthe average cell diameter L(av) was 190 μm. FIG. 2 shows a relationshipbetween the expansion ratio and the average cell diameter. Thefirst-stage expanded particles were subjected to in-mold expansionmolding after setting the internal air pressure at approximately 0.2 MPa(absolute pressure) through pressuring them with air in their respectivepressure-resistant closed vessels. The rate of fusion of each in-moldexpanded molded product was evaluated. The results are shown in Table 6.

In this comparative example, the average cell diameter varies widely inresponse to an increase in the expansion ratio. This shows that it isdifficult to control the expansion ratio without causing a significantchange in the average cell diameter. Further, it is shown that anincrease in the expansion ratio leads to degradation in fusibility ofthe in-mold expanded molded product.

INDUSTRIAL APPLICABILITY

Expanded polyolefin resin particles thus obtained can be processed intoin-mold expanded molded products of polyolefin resin by in-moldexpansion molding, which has conventionally been known. For example,such methods as follows can be used: (1) a method including subjectingthe expanded polyolefin resin particles to pressure treatment with aninorganic gas such as air or nitrogen, impregnating the pre-expandedparticles with an inorganic gas to impart a predetermined internalpressure to the pre-expanded particles, filling a mold with thepre-expanded particles, and fusing the pre-expanded particles by heat ofwater vapor; (2) a method including filling a mold with the expandedpolyolefin resin particles by compressing the expanded polyolefin resinparticles with gas pressure, fusing the expanded polyolefin resinparticles by heat of water vapor with use of the resilience of thepre-expanded particles; and (3) a method including filling a mold withthe expanded polyolefin resin particles without particular pretreatmentand fusing the expanded polyolefin resin particles by heat of watervapor.

1. (canceled)
 2. A process for producing expanded polyolefin resinparticles with use as a foaming agent of water contained in an aqueousdispersion medium, the process including the steps of: dispersingpolyolefin resin particles together with the aqueous dispersion mediuminto a closed vessel; heating the polyolefin resin particles up to orabove a softening temperature of the polyolefin resin particles andpressurizing the polyolefin resin particles; and releasing thepolyolefin resin particles into a zone whose pressure is lower than aninternal pressure of the closed vessel, the polyolefin resin particlesbeing composed of a polyolefin resin composition comprising: polyolefinresin; polyvalent alcohol in not less than 0.05 parts by weight to notmore than 2 parts by weight to 100 parts by weight of the polyolefinresin, the polyvalent alcohol having a carbon number of not less than 3to not more than 6 and three or more hydroxyl groups; and a foamnucleating agent. 3-7. (canceled)
 8. The process as set forth in claim2, wherein the polyvalent alcohol having a carbon number of not lessthan 3 to not more than 6 and three or more hydroxyl groups is one ormore types selected from among glycerin, diglycerin, pentaerythritol,trimethylolpropane, sorbitol, and D-mannitol.
 9. The process as setforth in claim 2, wherein the polyvalent alcohol having a carbon numberof not less than 3 to not more than 6 and three or more hydroxyl groupsis glycerin.
 10. The process as set forth in claim 9, wherein theglycerin is added in not less than 0.05 parts by weight to not more than0.5 parts by weight to 100 parts by weight of the polyolefin resin.11-18. (canceled)
 19. Expanded polyolefin resin particles that areobtained by a process as set forth in claim 2, said expanded polyolefinresin particles containing not less than 0.05% by weight to not morethan 2% by weight of the polyvalent alcohol having a carbon number ofnot less than 3 to not more than 6 and three or more hydroxyl groups,said expanded polyolefin resin particles having an expansion ratio ofnot less than 10 times to not more than 45 times and an average celldiameter of not less than 50 μm to not more than 800 μm, said expandedpolyolefin resin particles having a crystal structure that exhibits twoor more melting points on a DSC curve that is obtained by raising atemperature of the expanded polyolefin resin particles from 40° C. to220° C. at a heating rate of 10° C./min in differential scanningcalorimetry. 20-27. (canceled)
 28. The process as set forth in claim 2,wherein the polyolefin resin particles are composed of a polyolefinresin composition containing the foam nucleating agent in not less than0.005 parts by weight to not more than 2 parts by weight to 100 parts byweight of the polyolefin resin.
 29. (canceled)
 30. The process as setforth in claim 2, said process involving concomitant use of carbondioxide as a foaming agent.
 31. (canceled)
 32. A polyolefin resinin-mold expanded molded product that is obtained by filling a mold withexpanded polyolefin resin particles as set forth in claim 19 and heatingthe expanded polyolefin resin particles.